Add about_flag for experimenting MediaSession on Desktop

content/browser/media/session/pepper_playback_observer.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/browser/media/session/pepper_playback_observer.h"
 
 #include "base/feature_list.h"
 #include "base/metrics/histogram_macros.h"
 #include "content/browser/media/session/media_session.h"
 #include "content/browser/media/session/pepper_player_delegate.h"
@@ skipping 28 matching lines @@
   UMA_HISTOGRAM_BOOLEAN("Media.Pepper.PlayedSound",
                         players_played_sound_map_[pp_instance]);
   players_played_sound_map_.erase(pp_instance);
 
   PepperStopsPlayback(pp_instance);
 }
 
 void PepperPlaybackObserver::PepperStartsPlayback(int32_t pp_instance) {
   players_played_sound_map_[pp_instance] = true;
 
-  if (!base::FeatureList::IsEnabled(media::kFlashJoinsMediaSession))

[Alexei Svitkine (slow), 2016/10/18 16:31:32]

If you do end up going with the flag, rather than the feature, please also
remove the feature.

[Zhiqiang Zhang (Slow), 2016/10/18 16:46:26]

Done.

+  if (base::CommandLine::ForCurrentProcess()->GetSwitchValueASCII(
+          switches::kEnableDefaultMediaSession) !=
+      switches::kEnableDefaultMediaSessionWithFlash) {
     return;
+  }
 
   if (players_map_.count(pp_instance))
     return;
 
   players_map_[pp_instance].reset(new PepperPlayerDelegate(
       contents_, pp_instance));
   MediaSession::Get(contents_)->AddPlayer(
       players_map_[pp_instance].get(),
       PepperPlayerDelegate::kPlayerId,
       media::MediaContentType::Pepper);
 }
 
 void PepperPlaybackObserver::PepperStopsPlayback(int32_t pp_instance) {
   if (!players_map_.count(pp_instance))
     return;
 
   MediaSession::Get(contents_)->RemovePlayer(
       players_map_[pp_instance].get(), PepperPlayerDelegate::kPlayerId);
 
   players_map_.erase(pp_instance);
 }
 
 }  // namespace content