Introduce the concept of media task runner.

chromecast/media/media.gyp

 # 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.
 
 {
   'targets': [
     {
       'target_name': 'cma_base',
       'type': '<(component)',
       'dependencies': [
         '../../base/base.gyp:base',
         '../../media/media.gyp:media',
       ],
       'include_dirs': [
         '../..',
       ],
       'sources': [
+        'cma/base/balanced_media_task_runner_factory.cc',
+        'cma/base/balanced_media_task_runner_factory.h',
         'cma/base/buffering_controller.cc',
         'cma/base/buffering_controller.h',
         'cma/base/buffering_state.cc',
         'cma/base/buffering_state.h',
         'cma/base/cma_logging.h',
         'cma/base/decoder_buffer_adapter.cc',
         'cma/base/decoder_buffer_adapter.h',
         'cma/base/decoder_buffer_base.cc',
         'cma/base/decoder_buffer_base.h',
+        'cma/base/media_task_runner.cc',
+        'cma/base/media_task_runner.h',
       ],
     },
     {
       'target_name': 'cast_media',
       'type': 'none',
       'dependencies': [
         'cma_base',
       ],
     },
     {
       'target_name': 'cast_media_unittests',
       'type': '<(gtest_target_type)',
       'dependencies': [
         'cast_media',
         '../../base/base.gyp:base',
         '../../base/base.gyp:base_i18n',
         '../../base/base.gyp:test_support_base',
         '../../testing/gmock.gyp:gmock',
         '../../testing/gtest.gyp:gtest',
         '../../testing/gtest.gyp:gtest_main',
       ],
       'sources': [
+        'cma/base/balanced_media_task_runner_unittest.cc',
         'cma/base/buffering_controller_unittest.cc',
         'cma/base/run_all_unittests.cc',
       ],
     },
   ],
 }