CC Animation: Start replacing int layer_id with ElementId element_id.

cc/test/animation_timelines_test_common.h

 // Copyright 2015 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_TEST_ANIMATION_TIMELINES_TEST_COMMON_H_
 #define CC_TEST_ANIMATION_TIMELINES_TEST_COMMON_H_
 
 #include <memory>
 #include <unordered_map>
 
@@ skipping 215 matching lines @@
   int NextTestLayerId();
 
   TestHostClient client_;
   TestHostClient client_impl_;
 
   AnimationHost* host_;
   AnimationHost* host_impl_;
 
   const int timeline_id_;
   const int player_id_;
-  int layer_id_;
+  int element_id_;
 
   int next_test_layer_id_;
 
   scoped_refptr<AnimationTimeline> timeline_;
   scoped_refptr<AnimationPlayer> player_;
 
   scoped_refptr<AnimationTimeline> timeline_impl_;
   scoped_refptr<AnimationPlayer> player_impl_;
 };
 
 }  // namespace cc
 
 #endif  // CC_TEST_ANIMATION_TIMELINES_TEST_COMMON_H_