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

third_party/WebKit/Source/modules/compositorworker/AnimationWorkletMessagingProxy.cpp

 // 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.
 
 #include "modules/compositorworker/AnimationWorkletMessagingProxy.h"
 
 #include "core/workers/ThreadedWorkletObjectProxy.h"
 #include "modules/compositorworker/AnimationWorkletThread.h"
 
 namespace blink {
 
 AnimationWorkletMessagingProxy::AnimationWorkletMessagingProxy(
-    ExecutionContext* executionContext)
-    : ThreadedWorkletMessagingProxy(executionContext) {}
+    ExecutionContext* executionContext,
+    AnimationWorkletProxyClient* proxyClient)
+    : ThreadedWorkletMessagingProxy(executionContext),
+      m_proxyClient(proxyClient) {}
 
 AnimationWorkletMessagingProxy::~AnimationWorkletMessagingProxy() {}
 
 std::unique_ptr<WorkerThread>
 AnimationWorkletMessagingProxy::createWorkerThread(double originTime) {
   return AnimationWorkletThread::create(loaderProxy(), workletObjectProxy(),
                                         getParentFrameTaskRunners());
 }
 
 }  // namespace blink