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

third_party/WebKit/Source/web/CompositorMutatorImpl.h

Index: third_party/WebKit/Source/web/CompositorMutatorImpl.h
diff --git a/third_party/WebKit/Source/web/CompositorMutatorImpl.h b/third_party/WebKit/Source/web/CompositorMutatorImpl.h
index 3cf1f18e71aed1961d1f2bc7a828f5f88d4ee20d..6fb44605c12162d5efabb2fd01d7ca8fd326ab44 100644
--- a/third_party/WebKit/Source/web/CompositorMutatorImpl.h
+++ b/third_party/WebKit/Source/web/CompositorMutatorImpl.h
@@ -15,13 +15,14 @@
 namespace blink {
 
 class CompositorProxy;
-class CompositorProxyClientImpl;
+class CompositorAnimator;
 class CompositorMutatorClient;
 
-// Fans out requests from the compositor to all of the registered ProxyClients
-// which can then mutate layers through their CompositorProxy interfaces.
-// Requests for animation frames are received from ProxyClients and sent to the
-// compositor to generate a new compositor frame.
+// Fans out requests from the compositor to all of the registered
+// CompositorAnimators which can then mutate layers through their respective
+// mutate interface. Requests for animation frames are received from
+// CompositorAnimators and sent to the compositor to generate a new compositor
+// frame.
 //
 // Should be accessed only on the compositor thread.
 class CompositorMutatorImpl final : public CompositorMutator {
@@ -35,8 +36,8 @@ class CompositorMutatorImpl final : public CompositorMutator {
   bool mutate(double monotonicTimeNow,
               CompositorMutableStateProvider*) override;
 
-  void registerProxyClient(CompositorProxyClientImpl*);
-  void unregisterProxyClient(CompositorProxyClientImpl*);
+  void registerCompositorAnimator(CompositorAnimator*);
+  void unregisterCompositorAnimator(CompositorAnimator*);
 
   void setNeedsMutate();
 
@@ -48,9 +49,9 @@ class CompositorMutatorImpl final : public CompositorMutator {
  private:
   CompositorMutatorImpl();
 
-  using ProxyClients =
-      HashSet<CrossThreadPersistent<CompositorProxyClientImpl>>;
-  ProxyClients m_proxyClients;
+  using CompositorAnimators =
+      HashSet<CrossThreadPersistent<CompositorAnimator>>;
+  CompositorAnimators m_animators;
 
   std::unique_ptr<CustomCompositorAnimationManager> m_animationManager;
   CompositorMutatorClient* m_client;