Introduce the concept of media task runner.

chromecast/media/media.gyp

Index: chromecast/media/media.gyp
diff --git a/chromecast/media/media.gyp b/chromecast/media/media.gyp
index 6f7662d471f335bacc3a406d27bb92cddef96285..75f0547d30fcd720ad78e20cbbce1f128e42baa3 100644
--- a/chromecast/media/media.gyp
+++ b/chromecast/media/media.gyp
@@ -15,6 +15,8 @@
         '../..',
       ],
       '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',
@@ -24,6 +26,8 @@
         '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',
       ],
     },
     {
@@ -46,6 +50,7 @@
         '../../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',
       ],