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

cc/trees/single_thread_proxy.cc

Index: cc/trees/single_thread_proxy.cc
diff --git a/cc/trees/single_thread_proxy.cc b/cc/trees/single_thread_proxy.cc
index 38d23a2c60d01d9fcc7bf6e0b67c269aab35e481..5ee37f30595dc4960f5b2efb48a481c3867d6c27 100644
--- a/cc/trees/single_thread_proxy.cc
+++ b/cc/trees/single_thread_proxy.cc
@@ -364,6 +364,12 @@ void SingleThreadProxy::Stop() {
   layer_tree_host_ = NULL;
 }
 
+void SingleThreadProxy::SetMutator(std::unique_ptr<LayerTreeMutator> mutator) {
+  DCHECK(task_runner_provider_->IsMainThread());
+  DebugScopedSetImplThread impl(task_runner_provider_);
+  layer_tree_host_impl_->SetLayerTreeMutator(std::move(mutator));
+}
+
 void SingleThreadProxy::OnCanDrawStateChanged(bool can_draw) {
   TRACE_EVENT1(
       "cc", "SingleThreadProxy::OnCanDrawStateChanged", "can_draw", can_draw);