Windows paths use backward slash separator, which
escapes regular expressions for white and black lists
in PRESUBMIT.py filters

See https://goo.gl/6c7aku for discussion

Bug:

Change-Id: Ia1d133c5b6cfa6cb364b1500b7b7abe583bce346
Reviewed-on: https://chromium-review.googlesource.com/461141
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>