Prepare for responsive CSS animations.

Source/core/animation/CompositorAnimations.cpp

Index: Source/core/animation/CompositorAnimations.cpp
diff --git a/Source/core/animation/CompositorAnimations.cpp b/Source/core/animation/CompositorAnimations.cpp
index fb26b5784814040ede48647c98335ef5168361ed..d2b2185f73c1fad167dd223dc26bd724237063f0 100644
--- a/Source/core/animation/CompositorAnimations.cpp
+++ b/Source/core/animation/CompositorAnimations.cpp
@@ -72,6 +72,10 @@ void getKeyframeValuesForProperty(const KeyframeEffectModelBase* effect, CSSProp
 
 }
 
+CSSPropertyID CompositorAnimations::CompositableProperties[3] = {
+    CSSPropertyOpacity, CSSPropertyTransform, CSSPropertyWebkitFilter
+};
+
 bool CompositorAnimations::getAnimatedBoundingBox(FloatBox& box, const AnimationEffect& effect, double minValue, double maxValue) const
 {
     const KeyframeEffectModelBase& keyframeEffect = toKeyframeEffectModelBase(effect);
@@ -147,7 +151,8 @@ bool CompositorAnimations::isCandidateForAnimationOnCompositor(const Timing& tim
         ASSERT(keyframes.size() >= 2);
         for (const auto& keyframe : keyframes) {
             // FIXME: Determine candidacy based on the CSSValue instead of a snapshot AnimatableValue.
-            if (keyframe->composite() != AnimationEffect::CompositeReplace || !keyframe->getAnimatableValue())
+            bool isNeutralKeyframe = keyframe->isStringPropertySpecificKeyframe() && !toStringPropertySpecificKeyframe(keyframe.get())->value() && keyframe->composite() == AnimationEffect::CompositeAdd;
+            if ((keyframe->composite() != AnimationEffect::CompositeReplace && !isNeutralKeyframe) || !keyframe->getAnimatableValue())

[dstockwell, 2015/02/05 23:04:52]

When can getAnimatableValue() return null here?

[shend, 2015/02/06 03:16:25]

Since we're using StringKeyframes, initially the animatable values for all the
keyframes are null. For performance, we only snapshot animatable values for
compositable properties. So any keyframe that has non-compositable properties
will have a null animatable value (with the exception of keyframes using
LegacyStyleInterpolation). 

                 return false;
 
             switch (property) {