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

third_party/WebKit/Source/core/dom/CompositorWorkerProxyClient.cpp

Index: third_party/WebKit/Source/core/dom/CompositorWorkerProxyClient.cpp
diff --git a/third_party/WebKit/Source/core/dom/CompositorWorkerProxyClient.cpp b/third_party/WebKit/Source/core/dom/CompositorWorkerProxyClient.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..fe35790f71b4876a9eba19f3dc07828d7cbacbfd
--- /dev/null
+++ b/third_party/WebKit/Source/core/dom/CompositorWorkerProxyClient.cpp
@@ -0,0 +1,25 @@
+// 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 "core/dom/CompositorWorkerProxyClient.h"
+
+namespace blink {
+
+CompositorWorkerProxyClient* CompositorWorkerProxyClient::from(
+    WorkerClients* clients) {
+  return static_cast<CompositorWorkerProxyClient*>(
+      Supplement<WorkerClients>::from(clients, supplementName()));
+}
+
+const char* CompositorWorkerProxyClient::supplementName() {
+  return "CompositorWorkerProxyClient";
+}
+
+void provideCompositorWorkerProxyClientTo(WorkerClients* clients,
+                                          CompositorWorkerProxyClient* client) {
+  clients->provideSupplement(CompositorWorkerProxyClient::supplementName(),
+                             client);
+}
+
+}  // namespace blink