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

third_party/WebKit/Source/modules/compositorworker/AnimationWorkletThread.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/AnimationWorkletThread.h"
 
 #include "core/workers/WorkerThreadStartupData.h"
 #include "modules/compositorworker/AnimationWorkletGlobalScope.h"
 #include "platform/instrumentation/tracing/TraceEvent.h"
 #include "platform/weborigin/SecurityOrigin.h"
@@ skipping 24 matching lines @@
   TRACE_EVENT0(TRACE_DISABLED_BY_DEFAULT("animation-worklet"),
                "AnimationWorkletThread::createWorkerGlobalScope");
 
   RefPtr<SecurityOrigin> securityOrigin =
       SecurityOrigin::create(startupData->m_scriptURL);
   if (startupData->m_starterOriginPrivilegeData)
     securityOrigin->transferPrivilegesFrom(
         std::move(startupData->m_starterOriginPrivilegeData));
 
   // TODO(ikilpatrick): The AnimationWorkletGlobalScope will need to store a
-  // WorkerClients object for using a CompositorProxyClient object.
+  // WorkerClients object for using a CompositorWorkerProxyClient object.
   return AnimationWorkletGlobalScope::create(
       startupData->m_scriptURL, startupData->m_userAgent,
       securityOrigin.release(), this->isolate(), this);
 }
 
 }  // namespace blink