Add about_flag for experimenting MediaSession on Desktop

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 f9ade75b07fe10ad0a4d0f10a925af69dc8e2892..e43b97ad27faf73dc440e94e56096b5c41fa443b 100644
--- a/media/base/media_switches.cc
+++ b/media/base/media_switches.cc
@@ -71,7 +71,12 @@ const char kUseCras[] = "use-cras";
 // each other. This is different from the Media Session API as it is enabling a
 // default behaviour for the browser.
 const char kEnableDefaultMediaSession[] = "enable-default-media-session";
-#endif
+
+#if defined(ENABLE_PLUGINS)
+const char kEnableDefaultMediaSessionWithFlash[] = "with-flash";
+#endif  // defined(ENABLE_PLUGINS)
+
+#endif  // !defined(OS_ANDROID)
 
 // Use fake device for Media Stream to replace actual camera and microphone.
 const char kUseFakeDeviceForMediaStream[] = "use-fake-device-for-media-stream";