Fix ph error on about:tracing startup.

chrome/browser/resources/tracing/timeline_model.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 TimelineModel is a parsed representation of the
  * TraceEvents obtained from base/trace_event in which the begin-end
  * tokens are converted into a hierarchy of processes, threads,
  * subrows, and slices.
@@ skipping 484 matching lines @@
   var importerConstructors = [];
 
   /**
    * Registers an importer. All registered importers are considered
    * when processing an import request.
    *
    * @param {Function} importerConstructor The importer's constructor function.
    */
   TimelineModel.registerImporter = function(importerConstructor) {
     importerConstructors.push(importerConstructor);
-  }
+  };
+
+  function TimelineModelEmptyImporter(events) {
+  };
+
+  TimelineModelEmptyImporter.canImport = function(eventData) {
+    if (eventData instanceof Array && eventData.length == 0)
+      return true;
+    if (typeof(eventData) === 'string' || eventData instanceof String) {
+      return eventData.length == 0;
+    }
+    return false;
+  };
+
+  TimelineModelEmptyImporter.prototype = {
+    __proto__: Object.prototype,
+
+    importEvents : function() {
+    }
+  };
+
+  TimelineModel.registerImporter(TimelineModelEmptyImporter);
 
   TimelineModel.prototype = {
     __proto__: cr.EventTarget.prototype,
 
     get numProcesses() {
       var n = 0;
       for (var p in this.processes)
         n++;
       return n;
     },
@@ skipping 264 matching lines @@
     getStringHash: getStringHash,
     getStringColorId: getStringColorId,
 
     TimelineSlice: TimelineSlice,
     TimelineThread: TimelineThread,
     TimelineCounter: TimelineCounter,
     TimelineProcess: TimelineProcess,
     TimelineCpu: TimelineCpu,
     TimelineModel: TimelineModel
   };
+
 });