Tested and no regression on WebGL conformance 1.0.3

BUG=angle:628
BUG=368874
TEST=http://www.khronos.org/registry/webgl/sdk/tests/conformance/glsl/misc/boolean_precision.html

Change-Id: Ie74616c3ab846eea19f4bd4a041fc0f00d55f151
Reviewed-on: https://chromium-review.googlesource.com/198884Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Zhenyao Mo <zmo@chromium.org>