Do a better job of detecting where there are transitions.

The current code thinks that every element has transitions.

components/style/matching.rs

@@ -298,7 +298,7 @@ impl<E: TElement> StyleSharingCandidateCache<E> {
         }
 
         let box_style = style.get_box();
-        if box_style.transition_property_count() > 0 {
+        if box_style.specifies_transitions() {
             debug!("Failing to insert to the cache: transitions");
             return;
         }

components/style/properties/gecko.mako.rs

@@ -1890,6 +1890,19 @@ fn static_assert() {
             self.gecko.mTransitions[0].mProperty = nsCSSPropertyID_eCSSPropertyExtra_no_properties;
         }
     }
+
+    /// Returns whether there are any transitions specified.
+    pub fn specifies_transitions(&self) -> bool {
+        use gecko_bindings::structs::nsCSSPropertyID::eCSSPropertyExtra_all_properties;
+        if self.gecko.mTransitionPropertyCount == 1 &&
+            self.gecko.mTransitions[0].mProperty == eCSSPropertyExtra_all_properties &&
+            self.gecko.mTransitions[0].mDuration.max(0.0) + self.gecko.mTransitions[0].mDelay <= 0.0f32 {
+            return false;
+        }
+
+        self.gecko.mTransitionPropertyCount > 0
+    }
+
     pub fn transition_property_at(&self, index: usize)
         -> longhands::transition_property::computed_value::SingleComputedValue {
         self.gecko.mTransitions[index].mProperty.into()

components/style/properties/properties.mako.rs

@@ -1509,6 +1509,12 @@ pub mod style_structs {
             pub fn specifies_animations(&self) -> bool {
                 self.animation_name_iter().any(|name| name.0 != atom!(""))
             }
+
+            /// Returns whether there are any transitions specified.
+            #[cfg(feature = "servo")]
+            pub fn specifies_transitions(&self) -> bool {
+                self.transition_property_count() > 0
+            }
         % endif
     }