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 63208b4e578c46ffa5a38107a9c5cfdb7cb290a3..47ef413954c3fa808d239e1c413d1ce243da73c5 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;
@@ -348,6 +349,8 @@ class CC_EXPORT LayerTreeHost : public MutatorHostClient {
     return needs_meta_info_recomputation_;
   }
 
+  void SetLayerTreeMutator(std::unique_ptr<LayerTreeMutator> mutator);
+
   Layer* LayerById(int id) const;
 
   void AddLayerShouldPushProperties(Layer* layer);