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 3168dcaeca10c07390b74a540bab63e5ce7ab6bc..ad88d3411ec1b3998949388733c1c2706cbad2ef 100644
--- a/third_party/WebKit/Source/web/WebFrameWidgetImpl.h
+++ b/third_party/WebKit/Source/web/WebFrameWidgetImpl.h
@@ -134,7 +134,9 @@ class WebFrameWidgetImpl final
   // WebFrameWidgetBase overrides:
   bool forSubframe() const override { return true; }
   void scheduleAnimation() override;
-  CompositorProxyClient* createCompositorProxyClient() override;
+  CompositorWorkerProxyClient* createCompositorWorkerProxyClient() override;
+  AnimationWorkletProxyClient* createAnimationWorkletProxyClient() override;
+
   WebWidgetClient* client() const override { return m_client; }
   void setRootGraphicsLayer(GraphicsLayer*) override;
   void setRootLayer(WebLayer*) override;
@@ -194,6 +196,8 @@ class WebFrameWidgetImpl final
 
   LocalFrame* focusedLocalFrameAvailableForIme() const;
 
+  CompositorMutatorImpl& mutator();
+
   WebWidgetClient* m_client;
 
   // WebFrameWidget is associated with a subtree of the frame tree,