According to the resolution of Khonos bug 10287, we shouldn't be
validating these precisions. Note that this is counter to the
current ESSL spec text, but there's an open action item to change
the spec language.

TEST=dEQP-GLES3.functional.shaders.linkage.uniform.block.differing_precision
BUG=angleproject:1088

Change-Id: Ib05dca792b89e57e90d3a5f9efdb3a202230f585
Reviewed-on: https://chromium-review.googlesource.com/310760Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>