Introduce AnimationWorkletProxyClient and necessary plumbing to get it in worklet messaging proxy.

third_party/WebKit/Source/web/WebFrameWidgetImpl.h

Index: third_party/WebKit/Source/web/WebFrameWidgetImpl.h
diff --git a/third_party/WebKit/Source/web/WebFrameWidgetImpl.h b/third_party/WebKit/Source/web/WebFrameWidgetImpl.h
index 688894938800120b7336fa0592a5cb64cb0b96df..f7143c5962175ce4a414f943cab9bed352115a81 100644
--- a/third_party/WebKit/Source/web/WebFrameWidgetImpl.h
+++ b/third_party/WebKit/Source/web/WebFrameWidgetImpl.h
@@ -136,7 +136,7 @@ class WebFrameWidgetImpl final
   // WebFrameWidgetBase overrides:
   bool forSubframe() const override { return true; }
   void scheduleAnimation() override;
-  CompositorProxyClient* createCompositorProxyClient() override;
+  CompositorProxyClientFactory* compositorProxyClientFactory() override;
   WebWidgetClient* client() const override { return m_client; }
   void setRootGraphicsLayer(GraphicsLayer*) override;
   void setRootLayer(WebLayer*) override;
@@ -215,6 +215,7 @@ class WebFrameWidgetImpl final
   // compositor thread. The LayerTreeHostImpl is indirectly owned by this
   // class so this pointer should be valid until this class is destructed.
   CrossThreadPersistent<CompositorMutatorImpl> m_mutator;
+  Member<CompositorProxyClientFactoryImpl> m_proxyClientFactory;
 
   WebLayerTreeView* m_layerTreeView;
   WebLayer* m_rootLayer;