Otherwise in the Chromium build gyp might have a wrong cwd and then ninja
will fail to find the files to compile.

BUG=angleproject:892

Change-Id: I6b7beef5ecad62cc345b49253cedbde0ac166c17
Reviewed-on: https://chromium-review.googlesource.com/273161Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>