Unskip UniformBufferTest.SimpleBufferChange,
which was skipped by mistake instead of this test in
https://chromium-review.googlesource.com/833287

BUG=angleproject:2287

Change-Id: Id79821b26ad38ed07d1193b717ad222df3bb58fe
Reviewed-on: https://chromium-review.googlesource.com/838262Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>