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

cc/trees/single_thread_proxy.h

Index: cc/trees/single_thread_proxy.h
diff --git a/cc/trees/single_thread_proxy.h b/cc/trees/single_thread_proxy.h
index 6f515299ce155a0eda2355e06d76c217b9f51e78..eaf3b4168d594670ff6801820caccacc720cd025 100644
--- a/cc/trees/single_thread_proxy.h
+++ b/cc/trees/single_thread_proxy.h
@@ -56,6 +56,7 @@ class CC_EXPORT SingleThreadProxy : public Proxy,
   void Start(
       std::unique_ptr<BeginFrameSource> external_begin_frame_source) override;
   void Stop() override;
+  void SetMutator(std::unique_ptr<LayerTreeMutator> mutator) override;
   bool SupportsImplScrolling() const override;
   bool MainFrameWillHappenForTesting() override;
   void SetChildrenNeedBeginFrames(bool children_need_begin_frames) override;