The base level is passed to shaders in a uniform block created
specifically for passing sampler metadata. This is done on feature levels
above 9_3, which treat samplers as indices to sampler arrays in shaders.

BUG=angleproject:596
TEST=angle_end2end_tests

Change-Id: I846f2fc195ab1fd884052824ffd3c1d65083c0fb
Reviewed-on: https://chromium-review.googlesource.com/322122Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>