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 eae3ad4f3e27809f8d4115883268c6f887ae39ff..345c85abcf3417db5b02e496151770e9024c8cb4 100644
--- a/cc/trees/proxy_main.cc
+++ b/cc/trees/proxy_main.cc
@@ -412,6 +412,11 @@ void ProxyMain::Stop() {
   started_ = false;
 }
 
+void ProxyMain::SetMutator(LayerTreeMutator* mutator) {
+  TRACE_EVENT0("compositor-worker", "ThreadProxy::SetMutator");
+  channel_main_->InitializeMutatorOnImpl(mutator);
+}
+
 bool ProxyMain::SupportsImplScrolling() const {
   return true;
 }