We should use the cross-platform os.path.join() to support
platforms other than Windows.

BUG=angle:577

Change-Id: I9d96fc1b13731a57ffcd97469632f0acedb8e5c7
Reviewed-on: https://chromium-review.googlesource.com/188879Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>