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 40a134405a14c22fd78c5af84589638fc0137bae..cd4dff7cbeb59c269887632d5e23de852790bccc 100644
--- a/third_party/WebKit/Source/web/WebViewImpl.h
+++ b/third_party/WebKit/Source/web/WebViewImpl.h
@@ -55,6 +55,7 @@
 #include "public/web/WebPageImportanceSignals.h"
 #include "public/web/WebView.h"
 #include "web/ChromeClientImpl.h"
+#include "web/CompositorProxyClientFactoryImpl.h"
 #include "web/ContextMenuClientImpl.h"
 #include "web/EditorClientImpl.h"
 #include "web/MediaKeysClientImpl.h"
@@ -486,7 +487,7 @@ class WEB_EXPORT WebViewImpl final
   void forceNextWebGLContextCreationToFail() override;
   void forceNextDrawingBufferCreationToFail() override;
 
-  CompositorProxyClient* createCompositorProxyClient();
+  CompositorProxyClientFactory* compositorProxyClientFactory();
   IntSize mainFrameSize();
   WebDisplayMode displayMode() const { return m_displayMode; }
 
@@ -716,6 +717,7 @@ class WEB_EXPORT WebViewImpl 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;
+  Persistent<CompositorProxyClientFactoryImpl> m_proxyClientFactory;
 
   Persistent<EventListener> m_popupMouseWheelEventListener;