CC Animation: Start replacing int layer_id with ElementId element_id.

cc/trees/mutator_host_client.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_TREES_MUTATOR_HOST_CLIENT_H_
 #define CC_TREES_MUTATOR_HOST_CLIENT_H_
 
 namespace gfx {
 class Transform;
 class ScrollOffset;
 }
 
 namespace cc {
 
 class FilterOperations;
 class Layer;
 
+using ElementId = int;
+
 enum class LayerTreeType { ACTIVE, PENDING };
 
 class MutatorHostClient {
  public:
   virtual bool IsLayerInTree(int layer_id, LayerTreeType tree_type) const = 0;
   virtual void SetMutatorsNeedCommit() = 0;
   virtual void SetMutatorsNeedRebuildPropertyTrees() = 0;
 
   virtual void SetLayerFilterMutated(int layer_id,
                                      LayerTreeType tree_type,
@@ skipping 14 matching lines @@
       LayerTreeType tree_type,
       bool is_animating) = 0;
 
   virtual void ScrollOffsetAnimationFinished() = 0;
   virtual gfx::ScrollOffset GetScrollOffsetForAnimation(int layer_id) const = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_MUTATOR_HOST_CLIENT_H_