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

cc/trees/proxy_impl.cc

Index: cc/trees/proxy_impl.cc
diff --git a/cc/trees/proxy_impl.cc b/cc/trees/proxy_impl.cc
index bca90cb82eb2852c4d7369794c5fcf9d5aae5ffd..57edf3e4187b79c2fea663e6ec9fd0b7a9754e93 100644
--- a/cc/trees/proxy_impl.cc
+++ b/cc/trees/proxy_impl.cc
@@ -123,6 +123,12 @@ ProxyImpl::~ProxyImpl() {
   smoothness_priority_expiration_notifier_.Shutdown();
 }
 
+void ProxyImpl::InitializeMutatorOnImpl(LayerTreeMutator* mutator) {
+  TRACE_EVENT0("cc,compositor-worker", "ProxyImpl::InitializeMutatorOnImpl");
+  DCHECK(IsImplThread());
+  layer_tree_host_impl_->SetLayerTreeMutator(mutator);
+}
+
 void ProxyImpl::UpdateTopControlsStateOnImpl(TopControlsState constraints,
                                              TopControlsState current,
                                              bool animate) {