In the shading language grammar, layout qualifiers may appear in loop
conditions and function declarations, but this is not valid ESSL 3.00
according to section 4.3.8.

TEST=angle_unittests, dEQP-GLES3.functional.shaders.*layout*
BUG=angleproject:1064

Change-Id: I73f805247bf6678b953f2e1d45a23391e9eacb96
Reviewed-on: https://chromium-review.googlesource.com/285040Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>