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

cc/trees/layer_tree_host.cc

Index: cc/trees/layer_tree_host.cc
diff --git a/cc/trees/layer_tree_host.cc b/cc/trees/layer_tree_host.cc
index 17b1e6b8301ff32ad5edb80b4d1a7a7e668f03e8..b58d3e0001699aa8ce0931975f2069ceb161d210 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -1282,6 +1282,10 @@ void LayerTreeHost::RecordFrameTimingEvents(
                                    std::move(main_frame_events));
 }
 
+void LayerTreeHost::SetLayerTreeMutator(LayerTreeMutator* mutator) {
+  proxy_->SetMutator(mutator);
+}
+
 Layer* LayerTreeHost::LayerById(int id) const {
   LayerIdMap::const_iterator iter = layer_id_map_.find(id);
   return iter != layer_id_map_.end() ? iter->second : NULL;