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

third_party/WebKit/Source/platform/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//testing/test.gni")
 import("//third_party/WebKit/public/public_features.gni")
 import("//third_party/WebKit/Source/build/scripts/scripts.gni")
@@ skipping 763 matching lines @@
     "graphics/Color.cpp",
     "graphics/Color.h",
     "graphics/ColorBehavior.cpp",
     "graphics/ColorBehavior.h",
     "graphics/ColorSpace.cpp",
     "graphics/ColorSpace.h",
     "graphics/ColorSpaceProfileData.cpp",
     "graphics/ColorSpaceProfileData.h",
     "graphics/CompositingReasons.cpp",
     "graphics/CompositingReasons.h",
+    "graphics/CompositorAnimator.h",
     "graphics/CompositorElementId.cpp",
     "graphics/CompositorElementId.h",
     "graphics/CompositorFilterOperations.cpp",
     "graphics/CompositorFilterOperations.h",
     "graphics/CompositorMutableState.cpp",
     "graphics/CompositorMutableState.h",
     "graphics/CompositorMutableStateProvider.cpp",
     "graphics/CompositorMutableStateProvider.h",
     "graphics/CompositorMutationsTarget.h",
     "graphics/CompositorMutator.h",
@@ skipping 1302 matching lines @@
     "//third_party/WebKit/Source:config",
     "//third_party/WebKit/Source:inside_blink",
   ]
 
   deps = [
     ":test_support",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }