Negating the minimum representable integer overflows, so it
has undefined behavior in C++. Handle this as a special case in the
code.

BUG=chromium:637050
TEST=angle_unittests

Change-Id: Ic6e6d638faddad9b70b5d1637bb4b42ef4f43784
Reviewed-on: https://chromium-review.googlesource.com/390551Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>