This adds support for 'hooks_os' in .gclient which runs the given hooks
only when the associated os is specifed in target_os.

Bug: 706592
Change-Id: If70e51e0e784f8a8c6e45b33f59605b883a16f6e
Reviewed-on: https://chromium-review.googlesource.com/503534Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>