Improve caption button behavior for video player.

content/app/strings/content_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!-- This file contains definitions of resources that will be translated for
 each locale. Specifically, these are UI strings that are used by content that
 need to be translated for each locale.-->
 
 <!-- Some of these strings and string descriptions were taken from
 WebKit/win/WebCoreLocalizedStrings.cpp so we include the original license
 below:
 
@@ skipping 868 matching lines @@
       <message name="IDS_MEDIA_OVERFLOW_MENU_PLAY" desc="Media controls overflow menu item label for a play button.">
         Play
       </message>
       <message name="IDS_MEDIA_OVERFLOW_MENU_PAUSE" desc="Media controls overflow menu item label for a pause button.">
         Pause
       </message>
       <message name="IDS_MEDIA_OVERFLOW_MENU_DOWNLOAD" desc="Media controls overflow menu item label for a download button.">
        Download
       </message>
       <message name="IDS_MEDIA_TRACKS_NO_LABEL" desc="Menu item label for a text track that has no name specified">
-        Unknown
+        Track <ph name="NUMBER">$1<ex>1</ex></ph>

[mlamouri (slow - plz ping), 2016/10/28 13:02:37]

Maybe mention that the argument is the track number in the list of tracks.

       </message>
 
       <message name="IDS_MEDIA_TRACKS_OFF" desc="Menu item label for a text track that represents disabling closed captions">
         Off
       </message>
 
       <message name="IDS_PLUGIN_INITIALIZATION_ERROR" desc="A message displayed when a plugin failed to load">
         Couldn't load plugin.
       </message>
 
     </messages>
   </release>
 </grit>