Default precision can only be specified for float, int and sampler types.
Default precision for int also applies to uint and uvec declarations.

BUG=angleproject:1221
TEST=angle_unittests

Change-Id: I31fdcde80da16e2ea8771838f7c1a6ab4e478194
Reviewed-on: https://chromium-review.googlesource.com/313314
Tryjob-Request: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>