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

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

Index: third_party/WebKit/Source/core/loader/EmptyClients.h
diff --git a/third_party/WebKit/Source/core/loader/EmptyClients.h b/third_party/WebKit/Source/core/loader/EmptyClients.h
index 09b2f8d33dc71554ddc5ddb9cc271584f6bf8de3..6b3c85074be2d40e16138fc4d822ad96283a1580 100644
--- a/third_party/WebKit/Source/core/loader/EmptyClients.h
+++ b/third_party/WebKit/Source/core/loader/EmptyClients.h
@@ -218,7 +218,13 @@ class CORE_EXPORT EmptyChromeClient : public ChromeClient {
   void annotatedRegionsChanged() override {}
   String acceptLanguages() override;
 
-  CompositorProxyClient* createCompositorProxyClient(LocalFrame*) override {
+  CompositorWorkerProxyClient* createCompositorWorkerProxyClient(
+      LocalFrame*) override {
+    return nullptr;
+  }
+
+  AnimationWorkletProxyClient* createAnimationWorkletProxyClient(
+      LocalFrame*) override {
     return nullptr;
   }