CC Animation: Use bitset instead of unordered_set in LayerAnimationController

cc/animation/target_property.h

 // Copyright (c) 2016 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 CC_ANIMATION_TARGET_PROPERTY_H_
 #define CC_ANIMATION_TARGET_PROPERTY_H_
 
 namespace cc {
 
 namespace TargetProperty {
 
 enum Type {
   TRANSFORM = 0,
   OPACITY,
   FILTER,
   SCROLL_OFFSET,
   BACKGROUND_COLOR,
-  // This sentinel must be last.
+  // These sentinels must be last
+  FIRST_TARGET_PROPERTY = TRANSFORM,
   LAST_TARGET_PROPERTY = BACKGROUND_COLOR
 };
 
 const char* GetName(TargetProperty::Type property);
 
 }  // namespace TargetProperty
 
 }  // namespace cc
 
 #endif  // CC_ANIMATION_TARGET_PROPERTY_H_