Before this change, when running gn.py outside of
the checkout, the error was:
AttributeError: 'NoneType' object has no attribute 'endswith'

R=scottmg@chromium.org
Bug: gn:14

Change-Id: I4f2f83b2a861bd6f9436c16c2c422fe18fab8f2a
Reviewed-on: https://chromium-review.googlesource.com/c/1299023Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Lowell Manners <lowell@google.com>