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..f0047583981bdc2487eed3d97c9f3fc49a2179c6 100644
--- a/cc/trees/proxy_impl.cc
+++ b/cc/trees/proxy_impl.cc
@@ -123,6 +123,13 @@ ProxyImpl::~ProxyImpl() {
   smoothness_priority_expiration_notifier_.Shutdown();
 }
 
+void ProxyImpl::InitializeMutatorOnImpl(
+    std::unique_ptr<LayerTreeMutator> mutator) {
+  TRACE_EVENT0("cc,compositor-worker", "ProxyImpl::InitializeMutatorOnImpl");
+  DCHECK(IsImplThread());
+  layer_tree_host_impl_->SetLayerTreeMutator(std::move(mutator));
+}
+
 void ProxyImpl::UpdateTopControlsStateOnImpl(TopControlsState constraints,
                                              TopControlsState current,
                                              bool animate) {