In WebGL mode turn on checking that precision qualifiers match in
corresponding vertex/fragment uniform blocks.  Add test for this behavior.

BUG=angleproject:2015

Change-Id: Ie035138e8c46e430bdcf8fb5bbc3e0a3ab7d6391
Reviewed-on: https://chromium-review.googlesource.com/517724
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>