AMD driver in Linux requires invariant qualifier to match between
shaders even for GLSL >= 4.2. This conflicts with ESSL 3.0 which
disallows invariant qualifier in fragment shader. Remove invariant
qualifier in vertex shader to workaround AMD driver bug.

BUG=chromium:639760
TEST=webgl2_conformance

Change-Id: Id5adf7e7032105486df90a1f200471ea81ee5c36
Reviewed-on: https://chromium-review.googlesource.com/411917Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>