Negative operator is buggy in an expression on Intel Mac. Use abs(v) to
replace -v for negative value.

BUG=chromium:644057
TEST=deqp/functional/gles3/shaderpackingfunction.html

Change-Id: I6182e7a00b162e36ed9278c0e5a2fdd8fb480959
Reviewed-on: https://chromium-review.googlesource.com/381152
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>