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

cc/trees/proxy_main.h

Index: cc/trees/proxy_main.h
diff --git a/cc/trees/proxy_main.h b/cc/trees/proxy_main.h
index 29a7113209ea9235a7920b4a03cc04080329b27c..ea223cc6d7d7891f584eb986aed5a6f08387f1f4 100644
--- a/cc/trees/proxy_main.h
+++ b/cc/trees/proxy_main.h
@@ -22,6 +22,7 @@ class AnimationEvents;
 class BeginFrameSource;
 class ChannelMain;
 class LayerTreeHost;
+class LayerTreeMutator;
 
 // This class aggregates all interactions that the impl side of the compositor
 // needs to have with the main side.
@@ -108,6 +109,7 @@ class CC_EXPORT ProxyMain : public Proxy {
       std::unique_ptr<BeginFrameSource> external_begin_frame_source) override;
   void Stop() override;
   bool SupportsImplScrolling() const override;
+  void SetMutator(std::unique_ptr<LayerTreeMutator> mutator) override;
   bool MainFrameWillHappenForTesting() override;
   void SetChildrenNeedBeginFrames(bool children_need_begin_frames) override;
   void SetAuthoritativeVSyncInterval(const base::TimeDelta& interval) override;