[Android] Add support for external sharding to the test runner.

build/android/pylib/local/device/local_device_test_run.py

 # Copyright 2014 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 fnmatch
 import imp
 import logging
 import posixpath
 import signal
 import thread
@@ skipping 159 matching lines @@
       # See local_device_gtest_run._ExtractTestsFromFilter()
       if name.endswith('*'):
         return any(fnmatch.fnmatch(n, name) and is_failure_result(t)
                    for n, t in all_test_results.iteritems())
       return is_failure_result(all_test_results.get(name))
 
     failed_tests = (t for t in tests if test_failed(self._GetUniqueTestName(t)))
 
     return [t for t in failed_tests if self._ShouldRetry(t)]
 
+  def _ApplyExternalSharding(self, tests, shard_index, total_shards):
+    return [
+        t for t in tests
+        if hash(self._GetUniqueTestName(t)) % total_shards == shard_index]
+
   def GetTool(self, device):
     if not str(device) in self._tools:
       self._tools[str(device)] = valgrind_tools.CreateTool(
           self._env.tool, device)
     return self._tools[str(device)]
 
   def _CreateShards(self, tests):
     raise NotImplementedError
 
   def _GetUniqueTestName(self, test):
     # pylint: disable=no-self-use
     return test
 
   def _ShouldRetry(self, test):
     # pylint: disable=no-self-use,unused-argument
     return True
 
   def _GetTests(self):
     raise NotImplementedError
 
   def _RunTest(self, device, test):
     raise NotImplementedError
 
   def _ShouldShard(self):
     raise NotImplementedError
 
 
 class NoTestsError(Exception):
   """Error for when no tests are found."""