cc: Derive draw opacity from property trees

cc/trees/property_tree_builder.h

Index: cc/trees/property_tree_builder.h
diff --git a/cc/trees/property_tree_builder.h b/cc/trees/property_tree_builder.h
index f949c9a430efeef6b826726224348dc74940655d..7f5f479ec1c9df43a84c92ac7889d22a285ce887 100644
--- a/cc/trees/property_tree_builder.h
+++ b/cc/trees/property_tree_builder.h
@@ -16,6 +16,8 @@ class LayerTreeHost;
 
 class PropertyTreeBuilder {
  public:
+  // Building an opacity tree is optional, and can be skipped by passing
+  // in a null |opacity_tree|.
   static void BuildPropertyTrees(Layer* root_layer,
                                  const Layer* page_scale_layer,
                                  float page_scale_factor,
@@ -23,7 +25,8 @@ class PropertyTreeBuilder {
                                  const gfx::Rect& viewport,
                                  const gfx::Transform& device_transform,
                                  TransformTree* transform_tree,
-                                 ClipTree* clip_tree);
+                                 ClipTree* clip_tree,
+                                 OpacityTree* opacity_tree);
 };
 
 }  // namespace cc