webrtc-internals: add useragent to JSON dump

this adds the user agent to the dump. This allows identifying
the chrome version from the dump.

BUG=

Review-Url: https://codereview.chromium.org/2543493003
Cr-Commit-Position: refs/heads/master@{#437876}

content/browser/resources/media/dump_creator.js

@@ -118,6 +118,7 @@ var DumpCreator = (function() {
       {
         'getUserMedia': userMediaRequests,
         'PeerConnections': peerConnectionDataStore,
+        'UserAgent': navigator.userAgent,
       };
       var textBlob = new Blob([JSON.stringify(dump_object, null, ' ')],
                               {type: 'octet/stream'});