Local variables in GLSL can't use any other storage qualifier than
'const', but we weren't checking properly. For example, the can't
have 'in' or 'out' storage. This was causing a dEQP conformance
test failure.

BUG=angle:593

Change-Id: Ibb333950f2d0990a47d1a96caecc455497d5e57e
Reviewed-on: https://chromium-review.googlesource.com/191460Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>