blimp: Cleanup class/struct forward declarations

blimp/engine/app/blimp_system_url_request_context_getter.h

 // Copyright 2016 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 BLIMP_ENGINE_APP_BLIMP_SYSTEM_URL_REQUEST_CONTEXT_GETTER_H_
 #define BLIMP_ENGINE_APP_BLIMP_SYSTEM_URL_REQUEST_CONTEXT_GETTER_H_
 
 #include "base/files/file_path.h"
 #include "base/macros.h"
 #include "content/public/browser/content_browser_client.h"
 #include "net/url_request/url_request_context_getter.h"
 
 namespace base {
-class MessageLoop;
 class SingleThreadTaskRunner;
 }
 
 namespace blimp {
 namespace engine {
 
 // The URLRequestContextGetter for Blimp system requests that should not be
 // associated with a user URL request context (e.g. metrics upload).
 class BlimpSystemURLRequestContextGetter : public net::URLRequestContextGetter {
  public:
   BlimpSystemURLRequestContextGetter();
 
   // net::URLRequestContextGetter implementation.
   net::URLRequestContext* GetURLRequestContext() override;
   scoped_refptr<base::SingleThreadTaskRunner> GetNetworkTaskRunner()
       const override;
 
  private:
   ~BlimpSystemURLRequestContextGetter() override;
 
   std::unique_ptr<net::URLRequestContext> url_request_context_;
 
   DISALLOW_COPY_AND_ASSIGN(BlimpSystemURLRequestContextGetter);
 };
 
 }  // namespace engine
 }  // namespace blimp
 
 #endif  // BLIMP_ENGINE_APP_BLIMP_SYSTEM_URL_REQUEST_CONTEXT_GETTER_H_