Swarming UI: Fix issue with negative pending time.

appengine/swarming/ui/res/imp/taskpage/task-page.html

 <!--
   Copyright 2016 The LUCI Authors. All rights reserved.
   Use of this source code is governed under the Apache License, Version 2.0
   that can be found in the LICENSE file.
 
   This in an HTML Import-able file that contains the definition
   of the following elements:
 
     <task-page>
 
@@ skipping 902 matching lines @@
       },
 
       _disambiguate: function(taskId, result) {
         if (!taskId.endsWith("0") || !result) {
           return false;
         }
         return result.try_number > 1;
       },
 
       _duration: function(start, end) {
-        return end.getTime() - start.getTime();
+        var duration = end.getTime() - start.getTime();
+        if (duration < 0) {
+          duration = 0;
+        }
+        return duration;
       },
 
       _empty: function(array) {
         return !array || array.length == 0;
       },
 
       _eq: function(a,b) {
         return a === b;
       },
 
@@ skipping 238 matching lines @@
 
         this._getJsonAsync("_other_running", "/api/swarming/v1/tasks/count", "_busyRunningCount", this._auth_headers, taskCountParams);
         // change running to pending
         taskCountParams.state = ["PENDING"];
         this._getJsonAsync("_other_pending", "/api/swarming/v1/tasks/count", "_busyPendingCount", this._auth_headers, taskCountParams);
       }
     });
   })();
   </script>
 </dom-module>