abs(i) where i is an integer returns unexpected result in Intel Mac.
This works around the issue by emulating abs(i) manually.

BUG=chromium:642227
TEST=deqp/functional/gles3/shadercommonfunction.html

Change-Id: I2a41e0f4bcb0766109d651e663283b1760468017
Reviewed-on: https://chromium-review.googlesource.com/377628
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>