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

cc/trees/proxy_main.cc

Index: cc/trees/proxy_main.cc
diff --git a/cc/trees/proxy_main.cc b/cc/trees/proxy_main.cc
index fff11b497ddfd787958b3413494352bc6e3d021e..f685944693d5ef629e5f62212aa4ddd8d7fd0398 100644
--- a/cc/trees/proxy_main.cc
+++ b/cc/trees/proxy_main.cc
@@ -408,6 +408,11 @@ void ProxyMain::Stop() {
   started_ = false;
 }
 
+void ProxyMain::SetMutator(std::unique_ptr<LayerTreeMutator> mutator) {
+  TRACE_EVENT0("compositor-worker", "ThreadProxy::SetMutator");
+  channel_main_->InitializeMutatorOnImpl(std::move(mutator));
+}
+
 bool ProxyMain::SupportsImplScrolling() const {
   return true;
 }