Returns empty dict for GetIOStats on CrOS platforms.

tools/telemetry/telemetry/core/platform/cros_platform_backend.py

 # Copyright (c) 2013 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.
 
 from telemetry.core.platform import linux_platform_backend
 
 
 class CrosPlatformBackend(linux_platform_backend.LinuxPlatformBackend):
 
   def __init__(self, cri):
@@ skipping 17 matching lines @@
 
   def _RunCommand(self, args):
     return self._cri.RunCmdOnDevice(args)[0]
 
   def _GetFileContents(self, filename):
     try:
       return self._cri.RunCmdOnDevice(['cat', filename])[0]
     except AssertionError:
       return ''
 
+  def GetIOStats(self, pid):
+    # There is no '/proc/<pid>/io' file on CrOS platforms
+    # Returns empty dict as it does in PlatformBackend.
+    return {}
+
   def GetOSName(self):
     return 'chromeos'