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 fefa7741e613cf68b8b2ce742a8a85e5bf327559..3d368918f861182dd76a5d2021a5d9f7c33953ea 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 SetAuthoritativeVSyncInterval(