Bug 1122414 part 2 - Return the transitionProperty from Animation.name for CSS transitions; r=jwatt

This is a bit awkward. We lazily set mName to the transition property and then
return it. The reasons for this approach are:

* We don't really want to eagerly fill in mName for all transitions since in
  99% of cases we'll never use it and this will lead to wasted allocations.

* The signature of Name() returns a const nsString reference. This is because
  Name() is used when building CSS Animations (to compare different copies of
  the same animation when updating). For that case we don't really want to
  generate unnecessary copies of nsString objects so we return a reference.
  However, that means for transitions as well we need to return a reference so
  we can't just generate a temporary string on-demand.

  As a result we also have to const-cast ourselves so we can update the mName
  member. We could make mName mutable but seeing as it's only set once, the
  const_cast seems more appropriate.

dom/animation/Animation.h

@@ -249,7 +249,7 @@ public:
     return mTiming;
   }
 
-  const nsString& Name() const {
+  virtual const nsString& Name() const {
     return mName;
   }
 

dom/animation/test/css-transitions/test_animation-effect-name.html

@@ -16,8 +16,9 @@ test(function(t) {
   div.style.transition = 'all 100s';
   div.style.left = '100px';
 
-  assert_equals(div.getAnimations()[0].source.effect.name, '',
-                'Animation effects for transitions have an empty name');
+  assert_equals(div.getAnimations()[0].source.effect.name, 'left',
+                'The name for the transitions corresponds to the property ' +
+                'being transitioned');
 }, 'Effect name for transitions');
 
 </script>

layout/style/nsTransitionManager.cpp

@@ -24,6 +24,7 @@
 #include "nsIFrame.h"
 #include "Layers.h"
 #include "FrameLayerBuilder.h"
+#include "nsCSSProps.h"
 #include "nsDisplayList.h"
 #include "nsStyleChangeList.h"
 #include "nsStyleSet.h"
@@ -39,6 +40,16 @@ using namespace mozilla;
 using namespace mozilla::layers;
 using namespace mozilla::css;
 
+const nsString&
+ElementPropertyTransition::Name() const
+{
+   if (!mName.Length()) {
+     const_cast<ElementPropertyTransition*>(this)->mName =
+       NS_ConvertUTF8toUTF16(nsCSSProps::GetStringValue(TransitionProperty()));
+   }
+   return dom::Animation::Name();
+}
+
 double
 ElementPropertyTransition::CurrentValuePortion() const
 {

layout/style/nsTransitionManager.h

@@ -42,6 +42,8 @@ struct ElementPropertyTransition : public dom::Animation
   virtual ElementPropertyTransition* AsTransition() { return this; }
   virtual const ElementPropertyTransition* AsTransition() const { return this; }
 
+  virtual const nsString& Name() const;
+
   nsCSSProperty TransitionProperty() const {
     MOZ_ASSERT(Properties().Length() == 1,
                "Transitions should have exactly one animation property. "