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

third_party/WebKit/Source/core/dom/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("dom") {
   split_count = 5
 
   sources = [
     "AXObjectCache.cpp",
     "AXObjectCache.h",
+    "AnimationWorkletProxyClient.h",
     "Attr.cpp",
     "AttributeCollection.h",
     "CDATASection.cpp",
     "CDATASection.h",
     "CSSSelectorWatch.cpp",
     "CharacterData.cpp",
     "ChildFrameDisconnector.cpp",
     "ChildFrameDisconnector.h",
     "ChildListMutationScope.cpp",
     "ChildListMutationScope.h",
     "ChildNodeList.cpp",
     "ChildNodeList.h",
     "ClassCollection.cpp",
     "ClassCollection.h",
     "ClientRect.cpp",
     "ClientRect.h",
     "ClientRectList.cpp",
     "ClientRectList.h",
     "CollectionIndexCache.h",
     "Comment.cpp",
     "Comment.h",
     "CompositorProxiedPropertySet.cpp",
     "CompositorProxiedPropertySet.h",
     "CompositorProxy.cpp",
     "CompositorProxy.h",
-    "CompositorProxyClient.cpp",
     "CompositorProxyClient.h",
+    "CompositorWorkerProxyClient.cpp",
+    "CompositorWorkerProxyClient.h",
     "ContainerNode.cpp",
     "ContextFeatures.cpp",
     "ContextFeatures.h",
     "ContextLifecycleNotifier.cpp",
     "ContextLifecycleNotifier.h",
     "ContextLifecycleObserver.cpp",
     "ContextLifecycleObserver.h",
     "DOMArrayBuffer.cpp",
     "DOMArrayBuffer.h",
     "DOMArrayBufferBase.h",
@@ skipping 355 matching lines @@
     "shadow/SlotAssignment.h",
     "shadow/SlotScopedTraversal.cpp",
     "shadow/SlotScopedTraversal.h",
   ]
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }