Improve the ways timestamps are displayed on chrome://media-internals

Currently the timestamps are displayed with only 2 digits for the
millisecond field if it is less than 100 ms, resulting in misleading
numbers (e.g. 00:00:09 62). Updating the display to always show 3
digits for the milliseconds, and adding a period before
(e.g. 00:00:09.062).

BUG=
TEST=tested locally

Change-Id: I37e2cf375ce4311623e6a30271f495ebd0fc4d74
Reviewed-on: https://chromium-review.googlesource.com/c/1256205Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: John Rummell <jrummell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595909}

content/browser/resources/media/util.js

@@ -30,17 +30,19 @@ var util = (function() {
     }
   };
   util.millisecondsToString = function(timeMillis) {
-    function pad(num) {
+    function pad(num, len) {
       num = num.toString();
-      if (num.length < 2) {
-        return '0' + num;
+      while (num.length < len) {
+        num = '0' + num;
       }
       return num;
     }
 
     var date = new Date(timeMillis);
-    return pad(date.getUTCHours()) + ':' + pad(date.getUTCMinutes()) + ':' +
-        pad(date.getUTCSeconds()) + ' ' + pad((date.getMilliseconds()) % 1000);
+    return pad(date.getUTCHours(), 2) + ':' +
+           pad(date.getUTCMinutes(), 2) + ':' +
+           pad(date.getUTCSeconds(), 2) + '.' +
+           pad((date.getMilliseconds()) % 1000, 3);
   };
 
   return util;