Fix PRESUMBIT_tests.py on Windows.

MockInputApi was returning MockFile objects instead of strings
from LocalPaths() API, which apparently trips up Windows Python
APIs, but not Unix ones.

Bug: 984360
Change-Id: I0a14cc07a9406d84d6b20d104d002fe67cc4ae86
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1706577Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678454}

PRESUBMIT_test_mocks.py

@@ -111,7 +111,7 @@ class MockInputApi(object):
     return found_in_white_list
 
   def LocalPaths(self):
-    return self.files
+    return [file.LocalPath() for file in self.files]
 
   def PresubmitLocalPath(self):
     return self.presubmit_local_path