[Android] Fix error in sharding of gtests.

The formula for splitting tests into |device_num| shards
has always been incorrect.

BUG=

Review URL: https://codereview.chromium.org/12033066

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@178462 0039d316-1c4b-4281-b951-d872f2087c98

build/android/run_tests.py

@@ -176,8 +176,7 @@ def CreateShardedTestRunner(self, device, index):
       A SingleTestRunner object.
     """
     device_num = len(self.attached_devices)
-    shard_size = (len(self.tests) + device_num - 1) / device_num
-    shard_test_list = self.tests[index * shard_size : (index + 1) * shard_size]
+    shard_test_list = self.tests[index::device_num]
     test_filter = ':'.join(shard_test_list) + self.gtest_filter
     return SingleTestRunner(
         device,