Let Flash join and be controlled by media session

media/base/media_switches.h

Index: media/base/media_switches.h
diff --git a/media/base/media_switches.h b/media/base/media_switches.h
index 104bb0371f516c4d091187cdcae0bea33f37bf43..8284afc8754043da65012b92afc1e0b1474d3e10 100644
--- a/media/base/media_switches.h
+++ b/media/base/media_switches.h
@@ -71,6 +71,11 @@ namespace media {
 
 // All features in alphabetical order. The features should be documented
 // alongside the definition of their values in the .cc file.
+
+#if defined(ENABLE_PLUGINS)
+MEDIA_EXPORT extern const base::Feature kFlashJoinsMediaSession;
+#endif  // defined(ENABLE_PLUGINS)
+
 MEDIA_EXPORT extern const base::Feature kNewAudioRenderingMixingStrategy;
 MEDIA_EXPORT extern const base::Feature kUseNewMediaCache;