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..ef8eae32c878033e16cae83621efde770474c7a1 100644
--- a/cc/trees/layer_tree_host.cc
+++ b/cc/trees/layer_tree_host.cc
@@ -1282,6 +1282,11 @@ void LayerTreeHost::RecordFrameTimingEvents(
                                    std::move(main_frame_events));
 }
 
+void LayerTreeHost::SetLayerTreeMutator(
+    std::unique_ptr<LayerTreeMutator> mutator) {
+  proxy_->SetMutator(std::move(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;