Add about_flag for experimenting MediaSession on Desktop

chrome/app/generated_resources.grd

Index: chrome/app/generated_resources.grd
diff --git a/chrome/app/generated_resources.grd b/chrome/app/generated_resources.grd
index b2fbf87e74bf369c78d8ff45008d497c6b6307a3..0d8d0b41a521da46ffb5e720b7335e3271e3e879 100644
--- a/chrome/app/generated_resources.grd
+++ b/chrome/app/generated_resources.grd
@@ -15525,6 +15525,25 @@ read aloud to screenreader users to announce that a completion is available.">
     <message name="IDS_FLAGS_EXPENSIVE_BACKGROUND_TIMER_THROTTLING_DESCRIPTION" desc="Description for the flag to enable expensive background timer throttling">
       Enables intervention to limit CPU usage of background timers to 1%.
     </message>
+
+    <!-- Default MediaSession flag -->
+    <if expr="not is_android">
+        <message name="IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_NAME" desc="Name for the flag to enable default MediaSession on desktop">
+            Enable <ph name="FEATURE_NAME">default MediaSession</ph>
+        </message>
+        <message name="IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_DESCRIPTION" desc="Desciption for the flag to enable default MediaSession on desktop">
+            Enables the default MediaSession for managing audio focus across tabs.
+        </message>
+        <message name="IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_DISABLED" desc="Option for disabling the default MediaSession">
+            Disabled
+        </message>
+        <message name="IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_ENABLED" desc="Option for enabling the default MediaSession">
+            Enabled
+        </message>
+        <message name="IDS_FLAGS_ENABLE_DEFAULT_MEDIA_SESSION_ENABLED_WITH_FLASH" desc="Option for enabling the default MediaSession with Flash">
+            Enabled (with Flash)
+        </message>
+    </if>
   </messages>
  </release>
 </grit>