Remove WebThemeEngine

sky/engine/testing/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.
 
 static_library("platform") {
   output_name = "sky_testing_platform"
 
   sources = [
     "platform_impl.cc",
     "platform_impl.h",
-    "webthemeengine_impl.cc",
-    "webthemeengine_impl.h",
     "webunittestsupport_impl.cc",
     "webunittestsupport_impl.h",
   ]
 
   deps = [
     "//base",
     "//net",
     "//sky/engine",
     "//ui/native_theme",
     "//url",
     "//skia",
   ]
 }