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

third_party/WebKit/Source/web/WebViewImpl.h

Index: third_party/WebKit/Source/web/WebViewImpl.h
diff --git a/third_party/WebKit/Source/web/WebViewImpl.h b/third_party/WebKit/Source/web/WebViewImpl.h
index f648bad5821d61114ab5a16616c68c213bc50c01..bdeca0c99cde6b16d75a23753557be4000152108 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.h
+++ b/third_party/WebKit/Source/web/WebViewImpl.h
@@ -484,7 +484,9 @@ class WEB_EXPORT WebViewImpl final
   void forceNextWebGLContextCreationToFail() override;
   void forceNextDrawingBufferCreationToFail() override;
 
-  CompositorProxyClient* createCompositorProxyClient();
+  CompositorWorkerProxyClient* createCompositorWorkerProxyClient();
+  AnimationWorkletProxyClient* createAnimationWorkletProxyClient();
+
   IntSize mainFrameSize();
   WebDisplayMode displayMode() const { return m_displayMode; }
 
@@ -599,6 +601,8 @@ class WEB_EXPORT WebViewImpl final
   LocalFrame* focusedLocalFrameInWidget() const;
   LocalFrame* focusedLocalFrameAvailableForIme() const;
 
+  CompositorMutatorImpl& mutator();
+
   WebViewClient* m_client;  // Can be 0 (e.g. unittests, shared workers, etc.)
   WebSpellCheckClient* m_spellCheckClient;