Added stub MediaSourcePlayer for developing behind the flag

media/base/media_switches.cc

Index: media/base/media_switches.cc
diff --git a/media/base/media_switches.cc b/media/base/media_switches.cc
index c42bd9ff0ec8ccdeed666c6b76cfa1e9b1f6428c..f6ec92d71213f4dadd0e00ee4ab2b0978de963f0 100644
--- a/media/base/media_switches.cc
+++ b/media/base/media_switches.cc
@@ -21,6 +21,10 @@ const char kIgnoreResolutionLimitsForAcceleratedVideoDecode[] =
 // Disables the infobar popup for accessing protected media identifier.
 const char kDisableInfobarForProtectedMediaIdentifier[] =
     "disable-infobar-for-protected-media-identifier";
+
+// Sets the MediaSource player that uses the separate media thread
+const char kEnableMediaThreadForMediaPlayback[] =
+    "enable-media-thread-for-media-playback";
 #endif
 
 #if defined(OS_LINUX) || defined(OS_FREEBSD) || defined(OS_SOLARIS)