The usual set uniform call is used to set initial sampler uniform
values based on binding layout qualifiers. This is the simplest way to
ensure that the uniform values are set correctly in both internal data
structures of the Program class and on the different backends. This is
done as one of the last steps of program linking.

TEST=dEQP-GLES31.functional.uniform_binding.sampler*
BUG=angleproject:1442

Change-Id: I2f58c98e175f6a10a90042050f4fcbea77ad97ac
Reviewed-on: https://chromium-review.googlesource.com/456597Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>