LibWeb: Don't extrapolate transition properties for unknown properties

If we don't recognize a given transition-property value as a known CSS
property (one that we know about, not necessarily an invalid one),
we should not extrapolate the other transition-foo values for it.

Fixes #1480

Tests/LibWeb/Text/expected/WebAnimations/transition-unknown-property.txt

@@ -0,0 +1 @@
+   PASS (didn't crash)

Tests/LibWeb/Text/input/WebAnimations/transition-unknown-property.html

@@ -0,0 +1,15 @@
+<style>
+#foo {
+    transition-property: filter;
+    transition-timing-function: linear;
+    transition-duration:.3s;
+}
+</style>
+<div id="foo"></div>
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        foo.offsetWidth;
+        println("PASS (didn't crash)");
+    });
+</script>

Userland/Libraries/LibWeb/CSS/StyleComputer.cpp

@@ -1220,8 +1220,13 @@ static void compute_transitioned_properties(StyleProperties const& style, DOM::E
     // FIXME: Implement transitioning for pseudo-elements
     (void)pseudo_element;
 
-    if (auto const source_declaration = style.transition_property_source(); source_declaration && element.computed_css_values()) {
+    auto const source_declaration = style.transition_property_source();
-        if (source_declaration != element.cached_transition_property_source()) {
+    if (!source_declaration)
+        return;
+    if (!element.computed_css_values())
+        return;
+    if (source_declaration == element.cached_transition_property_source())
+        return;
     // Reparse this transition property
     element.clear_transitions();
     element.set_cached_transition_property_source(*source_declaration);
@@ -1231,34 +1236,6 @@ static void compute_transitioned_properties(StyleProperties const& style, DOM::E
         ? transition_properties_value->as_value_list().values()
         : StyleValueVector { transition_properties_value };
 
-            auto normalize_transition_length_list = [&](PropertyID property, auto make_default_value) {
-                auto style_value = style.maybe_null_property(property);
-                StyleValueVector list;
-
-                if (!style_value || !style_value->is_value_list() || style_value->as_value_list().size() == 0) {
-                    auto default_value = make_default_value();
-                    for (size_t i = 0; i < transition_properties.size(); i++)
-                        list.append(default_value);
-                    return list;
-                }
-
-                auto const& value_list = style_value->as_value_list();
-                for (size_t i = 0; i < transition_properties.size(); i++)
-                    list.append(value_list.value_at(i, true));
-
-                return list;
-            };
-
-            auto delays = normalize_transition_length_list(
-                PropertyID::TransitionDelay,
-                [] { return TimeStyleValue::create(Time::make_seconds(0.0)); });
-            auto durations = normalize_transition_length_list(
-                PropertyID::TransitionDuration,
-                [] { return TimeStyleValue::create(Time::make_seconds(0.0)); });
-            auto timing_functions = normalize_transition_length_list(
-                PropertyID::TransitionTimingFunction,
-                [] { return EasingStyleValue::create(EasingStyleValue::CubicBezier::ease()); });
-
     Vector<Vector<PropertyID>> properties;
 
     for (size_t i = 0; i < transition_properties.size(); i++) {
@@ -1290,10 +1267,36 @@ static void compute_transitioned_properties(StyleProperties const& style, DOM::E
         properties.append(move(properties_for_this_transition));
     }
 
+    auto normalize_transition_length_list = [&properties, &style](PropertyID property, auto make_default_value) {
+        auto style_value = style.maybe_null_property(property);
+        StyleValueVector list;
+
+        if (!style_value || !style_value->is_value_list() || style_value->as_value_list().size() == 0) {
+            auto default_value = make_default_value();
+            for (size_t i = 0; i < properties.size(); i++)
+                list.append(default_value);
+            return list;
+        }
+
+        auto const& value_list = style_value->as_value_list();
+        for (size_t i = 0; i < properties.size(); i++)
+            list.append(value_list.value_at(i, true));
+
+        return list;
+    };
+
+    auto delays = normalize_transition_length_list(
+        PropertyID::TransitionDelay,
+        [] { return TimeStyleValue::create(Time::make_seconds(0.0)); });
+    auto durations = normalize_transition_length_list(
+        PropertyID::TransitionDuration,
+        [] { return TimeStyleValue::create(Time::make_seconds(0.0)); });
+    auto timing_functions = normalize_transition_length_list(
+        PropertyID::TransitionTimingFunction,
+        [] { return EasingStyleValue::create(EasingStyleValue::CubicBezier::ease()); });
+
     element.add_transitioned_properties(move(properties), move(delays), move(durations), move(timing_functions));
 }
-    }
-}
 
 // https://drafts.csswg.org/css-transitions/#starting
 void StyleComputer::start_needed_transitions(StyleProperties const& previous_style, StyleProperties& new_style, DOM::Element& element, Optional<Selector::PseudoElement::Type> pseudo_element) const