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 15abffff4893d938fc25b1ae030e3606a3e1bb45..501aaf5aa05c75f064af3759446d60167dc67049 100644
--- a/media/base/media_switches.h
+++ b/media/base/media_switches.h
@@ -73,6 +73,11 @@ 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;