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 c6f70301748ab048cd8c84587fc50d2951d2f3c3..0e7daced4074d3522326cc597b98933da3e93f26 100644
--- a/chromecast/media/media.gyp
+++ b/chromecast/media/media.gyp
@@ -15,11 +15,15 @@
         '../..',
       ],
       '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/media_task_runner.cc',
+        'cma/base/media_task_runner.h',
       ],
     },
     {
@@ -42,6 +46,7 @@
         '../../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',
       ],