Fix ph error on about:tracing startup.

chrome/browser/resources/tracing/trace_event_importer.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview TraceEventImporter imports TraceEvent-formatted data
  * into the provided timeline model.
  */
 cr.define('tracing', function() {
   function ThreadState(tid) {
@@ skipping 47 matching lines @@
     // May be encoded JSON. But we dont want to parse it fully yet.
     // Use a simple heuristic:
     //   - eventData that starts with [ are probably trace_event
     //   - eventData that starts with { are probably trace_event
     // May be encoded JSON. Treat files that start with { as importable by us.
     if (typeof(eventData) === 'string' || eventData instanceof String) {
       return eventData[0] == '{' || eventData[0] == '[';
     }
 
     // Might just be an array of events
-    if (eventData instanceof Array && eventData[0].ph)
+    if (eventData instanceof Array && eventData.length && eventData[0].ph)
       return true;
 
     // Might be an object with a traceEvents field in it.
     if (eventData.traceEvents)
       return eventData.traceEvents instanceof Array &&
           eventData.traceEvents[0].ph;
 
     return false;
   };
 
@@ skipping 238 matching lines @@
         this.autoCloseOpenSlices();
     }
   };
 
   tracing.TimelineModel.registerImporter(TraceEventImporter);
 
   return {
     TraceEventImporter: TraceEventImporter
   };
 });