[scheduler] Remove WebThreadImplForUtilityThread from public API.

content/utility/utility_blink_platform_impl.h

 // Copyright 2015 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.
 
 #ifndef CONTENT_UTILITY_UTILITY_BLINK_PLATFORM_IMPL_H_
 #define CONTENT_UTILITY_UTILITY_BLINK_PLATFORM_IMPL_H_
 
 #include <memory>
 
 #include "base/macros.h"
 #include "content/child/blink_platform_impl.h"
 
 namespace blink {
 namespace scheduler {
-class WebThreadImplForUtilityThread;
+class WebThreadBase;
 }
 }
 
 namespace content {
 
 class UtilityBlinkPlatformImpl : public BlinkPlatformImpl {
  public:
   UtilityBlinkPlatformImpl();
   ~UtilityBlinkPlatformImpl() override;
 
   // BlinkPlatformImpl implementation.
   blink::WebThread* CurrentThread() override;
 
  private:
-  std::unique_ptr<blink::scheduler::WebThreadImplForUtilityThread> main_thread_;
+  std::unique_ptr<blink::scheduler::WebThreadBase> main_thread_;
 
   DISALLOW_COPY_AND_ASSIGN(UtilityBlinkPlatformImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_UTILITY_UTILITY_BLINK_PLATFORM_IMPL_H_