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

third_party/WebKit/Source/core/loader/EmptyClients.h

 /*
  * Copyright (C) 2006 Eric Seidel (eric@webkit.org)
  * Copyright (C) 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
  * Copyright (C) 2012 Samsung Electronics. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
@@ skipping 197 matching lines @@
   void setHasScrollEventHandlers(bool) override {}
   bool hasScrollEventHandlers() const override { return false; }
 
   void setTouchAction(LocalFrame*, TouchAction) override {}
 
   void didAssociateFormControlsAfterLoad(LocalFrame*) override {}
 
   void annotatedRegionsChanged() override {}
   String acceptLanguages() override;
 
-  CompositorProxyClient* createCompositorProxyClient(LocalFrame*) override {
+  CompositorProxyClient* createCompositorProxyClient(
+      CompositorProxyClient::Type,
+      LocalFrame*) override {
     return nullptr;
   }
 
   void registerPopupOpeningObserver(PopupOpeningObserver*) override {}
   void unregisterPopupOpeningObserver(PopupOpeningObserver*) override {}
 
   std::unique_ptr<WebFrameScheduler> createFrameScheduler(
       BlameContext*) override;
 };
 
@@ skipping 192 matching lines @@
   ~EmptyContextMenuClient() override {}
   bool showContextMenu(const ContextMenu*, bool) override { return false; }
   void clearContextMenu() override {}
 };
 
 CORE_EXPORT void fillWithEmptyClients(Page::PageClients&);
 
 }  // namespace blink
 
 #endif  // EmptyClients_h