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-Original-Commit-Position: refs/heads/master@{#550159}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 5208b8e8bb6ba0038016d17abac6911f3067b088