GN: Allow XCode projects to use ICECC environment variables.

Without them XCode cannot compile generated projects with ICECC.

Bug: 831500
Change-Id: I3b50c730660c4e8a6a06b9fda340e20edb87e40a
Reviewed-on: https://chromium-review.googlesource.com/1006577
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Michal Pichlinski <mpichlinski@opera.com>
Cr-Commit-Position: refs/heads/master@{#550159}
1 file changed