Make MediaSession a runtime-enabled feature on Desktop.

content/browser/media/media_web_contents_observer.h

Index: content/browser/media/media_web_contents_observer.h
diff --git a/content/browser/media/media_web_contents_observer.h b/content/browser/media/media_web_contents_observer.h
index d902dc9608d40b80ac856590de24396884c16d09..93e21981fb79d756bd1c9eb6a3a7bbe002b8d08d 100644
--- a/content/browser/media/media_web_contents_observer.h
+++ b/content/browser/media/media_web_contents_observer.h
@@ -13,10 +13,12 @@
 
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
+#include "content/browser/media/session/media_session_controllers_manager.h"
 #include "content/common/content_export.h"
 #include "content/public/browser/web_contents_observer.h"
 
 namespace content {
+
 class PowerSaveBlocker;
 
 // This class manages all RenderFrame based media related managers at the
@@ -47,6 +49,11 @@ class CONTENT_EXPORT MediaWebContentsObserver : public WebContentsObserver {
     return !!video_power_save_blocker_;
   }
 
+ protected:
+  MediaSessionControllersManager* session_controllers_manager() {
+    return &session_controllers_manager_;
+  }
+
  private:
   void OnMediaDestroyed(RenderFrameHost* render_frame_host, int delegate_id);
   void OnMediaPaused(RenderFrameHost* render_frame_host,
@@ -91,6 +98,8 @@ class CONTENT_EXPORT MediaWebContentsObserver : public WebContentsObserver {
   scoped_ptr<PowerSaveBlocker> audio_power_save_blocker_;
   scoped_ptr<PowerSaveBlocker> video_power_save_blocker_;
 
+  MediaSessionControllersManager session_controllers_manager_;
+
   DISALLOW_COPY_AND_ASSIGN(MediaWebContentsObserver);
 };