Install warning message for extensions that use chrome.mdns API.

BUG=451526

Review URL: https://codereview.chromium.org/1012173002

Cr-Commit-Position: refs/heads/master@{#321500}

chrome/app/generated_resources.grd

@@ -4620,6 +4620,9 @@ Even if you have downloaded files from this website before, the website might ha
       <message name="IDS_EXTENSION_PROMPT_WARNING_MANAGEMENT" desc="Permisson string for access to extension management.">
         Manage your apps, extensions, and themes
       </message>
+      <message name="IDS_EXTENSION_PROMPT_WARNING_MDNS" desc="Permisson string for access to list of mDNS services on a network.">
+        Discover devices on your local network
+      </message>
       <message name="IDS_EXTENSION_PROMPT_WARNING_TOPSITES" desc="Permission string for access to the list of most frequently visited websites.">
         Read a list of your most frequently visited websites
       </message>