[Telemetry + tools/perf] Modify the pylint disable message to use symbolic name

tools/perf/fetch_benchmark_deps_unittest.py

 # Copyright 2015 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.
 
 import os
 import sys
 import unittest
 import StringIO
 
-import mock # pylint: disable=F0401
+import mock # pylint: disable=import-error
 
 import fetch_benchmark_deps
 
 
 def NormPaths(paths):
   return sorted([os.path.normcase(p) for p in paths.splitlines()])
 
 
 class FetchBenchmarkDepsUnittest(unittest.TestCase):
   """The test guards fetch_benchmark_deps.
@@ skipping 42 matching lines @@
 
   def testFetchServingDirs(self):
     self._RunFetchBenchmarkDepsTest('media.tough_video_cases')
 
   def testFetchOctane(self):
     octane_wpr_path = os.path.join(
         os.path.dirname(__file__), 'page_sets', 'data', 'octane_001.wpr')
     expected = os.path.relpath(octane_wpr_path,
                                fetch_benchmark_deps.GetChromiumDir())
     self._RunFetchBenchmarkDepsTest('octane', NormPaths(expected))