It was found that the driver issue only affects vertex
shaders, so the workaround can be simplified to affect only
vertex shaders.

BUG=angleproject:2294
TEST=WebGL conformance tests on passthrough command buffer,
     angle_end2end_tests

Change-Id: Ie7fb4e75a3999e1de3d5c20d3de21e7ebb08f148
Reviewed-on: https://chromium-review.googlesource.com/889099Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>