Make MediaSession a runtime-enabled feature on Desktop.

content/content_browser.gypi

Index: content/content_browser.gypi
diff --git a/content/content_browser.gypi b/content/content_browser.gypi
index 1953f4c5b0d2a89e87abc79283ea6ec32e58321c..349e91ab3d256bfb2060c2292c60f5677e74a5aa 100644
--- a/content/content_browser.gypi
+++ b/content/content_browser.gypi
@@ -1023,13 +1023,6 @@
       'browser/media/android/browser_surface_view_manager.h',
       'browser/media/android/media_resource_getter_impl.cc',
       'browser/media/android/media_resource_getter_impl.h',
-      'browser/media/android/media_session.cc',
-      'browser/media/android/media_session.h',
-      'browser/media/android/media_session_controller.cc',
-      'browser/media/android/media_session_controller.h',
-      'browser/media/android/media_session_observer.h',
-      'browser/media/android/media_session_uma_helper.cc',
-      'browser/media/android/media_session_uma_helper.h',
       'browser/media/android/media_throttler.cc',
       'browser/media/android/media_throttler.h',
       'browser/media/android/media_web_contents_observer_android.cc',
@@ -1070,6 +1063,19 @@
       'browser/media/media_web_contents_observer.h',
       'browser/media/midi_host.cc',
       'browser/media/midi_host.h',
+      'browser/media/session/media_session.cc',
+      'browser/media/session/media_session.h',
+      'browser/media/session/media_session_controller.cc',
+      'browser/media/session/media_session_controller.h',
+      'browser/media/session/media_session_controllers_manager.cc',
+      'browser/media/session/media_session_controllers_manager.h',
+      'browser/media/session/media_session_delegate.h',
+      'browser/media/session/media_session_delegate_android.h',
+      'browser/media/session/media_session_delegate_android.cc',
+      'browser/media/session/media_session_delegate_default.cc',
+      'browser/media/session/media_session_observer.h',
+      'browser/media/session/media_session_uma_helper.cc',
+      'browser/media/session/media_session_uma_helper.h',
       'browser/media/webrtc/webrtc_identity_store.cc',
       'browser/media/webrtc/webrtc_identity_store.h',
       'browser/media/webrtc/webrtc_identity_store_backend.cc',
@@ -2070,6 +2076,7 @@
         'browser/geolocation/device_data_provider.cc',
         'browser/geolocation/empty_device_data_provider.cc',
         'browser/geolocation/wifi_data_provider_common.cc',
+        'browser/media/session/media_session_delegate_default.cc',
         'browser/power_usage_monitor_impl.cc',
         'browser/power_usage_monitor_impl.h',
         'browser/renderer_host/begin_frame_observer_proxy.cc',