cc: Derive draw opacity from property trees

cc/trees/property_tree.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 CC_TREES_PROPERTY_TREE_H_
 #define CC_TREES_PROPERTY_TREE_H_
 
 #include <vector>
 
 #include "base/basictypes.h"
@@ skipping 82 matching lines @@
   ClipNodeData();
 
   gfx::RectF clip;
   gfx::RectF combined_clip;
   int transform_id;
   int target_id;
 };
 
 typedef TreeNode<ClipNodeData> ClipNode;
 
+typedef TreeNode<float> OpacityNode;
+
 template <typename T>
 class CC_EXPORT PropertyTree {
  public:
   PropertyTree();
   virtual ~PropertyTree();
 
   int Insert(const T& tree_node, int parent_id);
 
   T* Node(int i) { return i > -1 ? &nodes_[i] : nullptr; }
   const T* Node(int i) const { return i > -1 ? &nodes_[i] : nullptr; }
@@ skipping 62 matching lines @@
                                   TransformNode* target_node);
   void UpdateSublayerScale(TransformNode* node);
   void UpdateTargetSpaceTransform(TransformNode* node,
                                   TransformNode* target_node);
   void UpdateIsAnimated(TransformNode* node, TransformNode* parent_node);
   void UpdateSnapping(TransformNode* node);
 };
 
 class CC_EXPORT ClipTree final : public PropertyTree<ClipNode> {};
 
+class CC_EXPORT OpacityTree final : public PropertyTree<OpacityNode> {};
+
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_