Make the mute button in the Ash system menu keyboard focusable

ash/ash_strings.grd

 <?xml version="1.0" encoding="UTF-8"?>
 
 <!--
 This file contains the strings for ash.
 -->
 
 <grit base_dir="." latest_public_release="0" current_release="1"
       output_all_resource_defines="false" source_lang_id="en" enc_check="möl">
   <outputs>
     <output filename="grit/ash_strings.h" type="rc_header">
@@ skipping 265 matching lines @@
         Restart to update
       </message>
 
       <message name="IDS_ASH_STATUS_TRAY_RESTART_AND_POWERWASH_TO_UPDATE" desc="The label used in the tray popup to notify that the user should restart and powerwash the device to get system updates.">
         Restart and powerwash to update
       </message>
 
       <message name="IDS_ASH_STATUS_TRAY_VOLUME" desc="The accessible text for the volume slider.">
         Volume
       </message>
+      <message name="IDS_ASH_STATUS_TRAY_VOLUME_MUTE" desc="The accessible text for the volume mute button.">
+        Mute
+      </message>
 
       <message name="IDS_ASH_STATUS_TRAY_UNKNOWN_DISPLAY_NAME" desc="Label shown in tray for a display whose name is unknown.">
         Unknown Display
       </message>
       <message name="IDS_ASH_STATUS_TRAY_DRIVE" desc="The label used for Google Drive tray details header.">
         Google Drive
       </message>
       <message name="IDS_ASH_STATUS_TRAY_DRIVE_SETTINGS" desc="The label used for Google Drive settings entry.">
         Google Drive settings...
       </message>
@@ skipping 120 matching lines @@
 
       <!-- ChromeOS-specific strings -->
       <if expr="chromeos">
         <part file="ash_chromeos_strings.grdp" />
       </if>
 
     </messages>
   </release>
 </grit>