[Media Router WebUI] Update "Device missing?" messaging.

chrome/app/media_router_strings.grdp

 <?xml version="1.0" encoding="utf-8"?>
 <!-- Media Router-specific strings (included from generated_resources.grd). -->
 <grit-part>
   <!-- General -->
   <message name="IDS_MEDIA_ROUTER_TITLE" desc="Title of a dialog offering possible actions for Media Router, which enables users to mirror individual tabs or desktop as well as cast content to a device.">
     Media Router
   </message>
   <message name="IDS_MEDIA_ROUTER_ICON_TOOLTIP_TEXT" desc="Tooltip for the Media Router Action icon, which appears in the toolbar. The tooltip appears on mouseover of the icon.">
    Display on another screen
   </message>
@@ skipping 119 matching lines @@
     Casting: <ph name="routeTitle">$1<ex>Video</ex></ph>
   </message>
   <message name="IDS_MEDIA_ROUTER_START_CASTING_BUTTON" desc="Start casting button, which, on click, will start casting to the current route.">
     Cast
   </message>
   <message name="IDS_MEDIA_ROUTER_STOP_CASTING_BUTTON" desc="Stop casting button, which, on click, will stop the current route.">
     Stop
   </message>
 
   <!-- Sink List -->
-  <message name="IDS_MEDIA_ROUTER_DEVICE_MISSING" desc="Link to display when no devices are found which, on click, opens a page to the Chromecast help center explaining possible reasons why no devices are detected.">
-    Device missing?
+  <message name="IDS_MEDIA_ROUTER_DESTINATION_MISSING" desc="Link to display when no Cast destinations are found which, on click, opens a page to the Chromecast help center explaining possible reasons why none are detected.">
+    No Cast destinations found. Need help?
   </message>
 
   <!-- Sink Search -->
   <message name="IDS_MEDIA_ROUTER_SEARCH_LABEL" desc="Label for search input in sink list.">
     Search
   </message>
   <message name="IDS_MEDIA_ROUTER_SEARCH_NO_MATCHES" desc="Text to display when there are no sinks matching the user input search text.">
     No matches
   </message>
 </grit-part>