This avoid us needing to replicate gyp variables in both Chromium's
and ANGLE's common.gypi files.

BUG=angleproject:998

Change-Id: Iba1938fd37eb4bee28ae7daad31bf85a2d3bd692
Reviewed-on: https://chromium-review.googlesource.com/282145Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Kenneth Russell <kbr@chromium.org>