Use enum instead of string in MessageLoopFactory::GetMessageLoop*

media/base/pipeline.h

 // Copyright (c) 2012 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 MEDIA_BASE_PIPELINE_H_
 #define MEDIA_BASE_PIPELINE_H_
 
 #include <string>
 
 #include "base/gtest_prod_util.h"
@@ skipping 80 matching lines @@
 // playing the media.
 //
 // If any error ever happens, this object will transition to the "Error" state
 // from any state. If Stop() is ever called, this object will transition to
 // "Stopped" state.
 class MEDIA_EXPORT Pipeline
     : public base::RefCountedThreadSafe<Pipeline>,
       public DemuxerHost {
  public:
   // Constructs a media pipeline that will execute on |message_loop|.
-  Pipeline(MessageLoop* message_loop, MediaLog* media_log);
+  Pipeline(const scoped_refptr<base::MessageLoopProxy>& message_loop,
+           MediaLog* media_log);
 
   // Build a pipeline to using the given filter collection to construct a filter
   // chain, executing |start_cb| when initialization has completed.
   //
   // The following permanent callbacks will be executed as follows up until
   // Stop() has completed:
   //   |ended_cb| will be executed whenever the media reaches the end.
   //   |error_cb| will be executed whenever an error occurs but hasn't
   //              been reported already through another callback.
   //
@@ skipping 409 matching lines @@
   base::Time creation_time_;
 
   scoped_ptr<SerialRunner> pending_callbacks_;
 
   DISALLOW_COPY_AND_ASSIGN(Pipeline);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_BASE_PIPELINE_H_