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 d62340f58cf7f6a6fd75edad5ec2667566d61a01..c0792a6154190d084ca4f4a7feba73708127437c 100644
--- a/cc/trees/proxy_main.cc
+++ b/cc/trees/proxy_main.cc
@@ -404,6 +404,11 @@ 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;
 }