Add systemInfo.cpu.onUpdated event implementation.

chrome/common/extensions/api/experimental_system_info_cpu.idl

 // Copyright (c) 2012 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.
 
 // File-level comment to appease parser. Eventually this will not be necessary.
 namespace experimental.systemInfo.cpu {
 
   dictionary CpuInfo {
     // The number of logical processors.
     long numOfProcessors;
@@ skipping 10 matching lines @@
     // The CPU usage array for each logic processor.
     double[] usagePerProcessor;
   };
 
   callback CpuInfoCallback = void (CpuInfo info);
   
   interface Functions {
     // Get CPU information.
     static void get(CpuInfoCallback callback);
   };
+
+  interface Events {
+    // Fired periodically to report CPU history usage information. The default
+    // period interval is 1 seconds.
+    static void onUpdated(CpuUpdateInfo info);
+  };
 };