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

content/renderer/gpu/render_widget_compositor.h

Index: content/renderer/gpu/render_widget_compositor.h
diff --git a/content/renderer/gpu/render_widget_compositor.h b/content/renderer/gpu/render_widget_compositor.h
index 1e15b8d2727bdfd38d859b18f65bcbb75d5ed5ad..89678e94d0456bce409814a6247ea34541a9d98a 100644
--- a/content/renderer/gpu/render_widget_compositor.h
+++ b/content/renderer/gpu/render_widget_compositor.h
@@ -147,6 +147,9 @@ class CONTENT_EXPORT RenderWidgetCompositor
   void setShowDebugBorders(bool show) override;
   void setShowScrollBottleneckRects(bool show) override;
 
+  void setMutatorClient(
+      std::unique_ptr<blink::WebCompositorMutatorClient>) override;
+
   void updateTopControlsState(blink::WebTopControlsState constraints,
                               blink::WebTopControlsState current,
                               bool animate) override;