Add TransitionKeyframe class for CSS Transition refactor

third_party/WebKit/Source/core/animation/animatable/AnimatableValueKeyframe.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef AnimatableValueKeyframe_h
 #define AnimatableValueKeyframe_h
 
 #include "core/CoreExport.h"
 #include "core/animation/Keyframe.h"
 #include "core/animation/animatable/AnimatableValue.h"
 #include "core/css/CSSPropertyIDTemplates.h"
 
 namespace blink {
 
+// TODO(alancutter): Delete this class once TransitionKeyframe has completely
+// replaced it.
 class CORE_EXPORT AnimatableValueKeyframe : public Keyframe {
  public:
   static PassRefPtr<AnimatableValueKeyframe> create() {
     return adoptRef(new AnimatableValueKeyframe);
   }
   void setPropertyValue(CSSPropertyID property,
                         PassRefPtr<AnimatableValue> value) {
     m_propertyValues.set(property, std::move(value));
   }
   void clearPropertyValue(CSSPropertyID property) {
@@ skipping 73 matching lines @@
                   value.isAnimatableValueKeyframe());
 DEFINE_TYPE_CASTS(AnimatableValuePropertySpecificKeyframe,
                   Keyframe::PropertySpecificKeyframe,
                   value,
                   value->isAnimatableValuePropertySpecificKeyframe(),
                   value.isAnimatableValuePropertySpecificKeyframe());
 
 }  // namespace blink
 
 #endif