compositor-worker: Initialize CW machinery plumbing to compositor and fire CW rAF callbacks.

cc/trees/layer_tree_host.h

Index: cc/trees/layer_tree_host.h
diff --git a/cc/trees/layer_tree_host.h b/cc/trees/layer_tree_host.h
index 0573db3a70289fb00b0feaaf6f6cd814a7391ccf..0d2d0ae1febc6c819d88548d866621a0bfb15ef4 100644
--- a/cc/trees/layer_tree_host.h
+++ b/cc/trees/layer_tree_host.h
@@ -61,6 +61,7 @@ class Layer;
 class LayerTreeHostImpl;
 class LayerTreeHostImplClient;
 class LayerTreeHostSingleThreadClient;
+class LayerTreeMutator;
 class PropertyTrees;
 class Region;
 class RemoteProtoChannel;
@@ -357,6 +358,8 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
       std::unique_ptr<FrameTimingTracker::MainFrameTimingSet>
           main_frame_events);
 
+  void SetLayerTreeMutator(std::unique_ptr<LayerTreeMutator> mutator);
+
   Layer* LayerById(int id) const;
 
   void AddLayerShouldPushProperties(Layer* layer);