This is necessary for Chrome OS to calculate sunset time.

Bug:chromium:726654
Test: With https://codereview.chromium.org/2887913004 patched in, 'gn
Change-Id: Icf6a8cd833f8afcd1a2d0ce874cf1c8c92fcba44
gen <out_dir> --check' does not complain.
TBR=afakhry@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/520323


Reviewed-by: Jungshik Shin <jshin@chromium.org>