cc: Derive draw opacity from property trees

cc/trees/property_tree.h

Index: cc/trees/property_tree.h
diff --git a/cc/trees/property_tree.h b/cc/trees/property_tree.h
index ef47bce9edd1a1bb7938ac7771b895f7d79c64e4..404f498d4e3ca9ef4a0daeed0b2ca4101a756305 100644
--- a/cc/trees/property_tree.h
+++ b/cc/trees/property_tree.h
@@ -100,6 +100,8 @@ struct CC_EXPORT ClipNodeData {
 
 typedef TreeNode<ClipNodeData> ClipNode;
 
+typedef TreeNode<float> OpacityNode;
+
 template <typename T>
 class CC_EXPORT PropertyTree {
  public:
@@ -182,6 +184,8 @@ class CC_EXPORT TransformTree final : public PropertyTree<TransformNode> {
 
 class CC_EXPORT ClipTree final : public PropertyTree<ClipNode> {};
 
+class CC_EXPORT OpacityTree final : public PropertyTree<OpacityNode> {};
+
 }  // namespace cc
 
 #endif  // CC_TREES_PROPERTY_TREE_H_