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

cc/trees/proxy.h

Index: cc/trees/proxy.h
diff --git a/cc/trees/proxy.h b/cc/trees/proxy.h
index 5c39d7066a80125139394cd1e34f1a2b7737483e..4969d7d0c126dd5e750bf50628d423368bdac520 100644
--- a/cc/trees/proxy.h
+++ b/cc/trees/proxy.h
@@ -26,6 +26,7 @@ class Vector2d;
 namespace cc {
 class BeginFrameSource;
 class LayerTreeDebugState;
+class LayerTreeMutator;
 class OutputSurface;
 struct RendererCapabilities;
 
@@ -72,6 +73,8 @@ class CC_EXPORT Proxy {
       std::unique_ptr<BeginFrameSource> external_begin_frame_source) = 0;
   virtual void Stop() = 0;   // Must be called before deleting the proxy.
 
+  virtual void SetMutator(std::unique_ptr<LayerTreeMutator> mutator) = 0;
+
   virtual bool SupportsImplScrolling() const = 0;
 
   virtual void SetChildrenNeedBeginFrames(bool children_need_begin_frames) = 0;