If the uniform block will be translated to HLSL StructuredBuffer,
in order to follow the std140 rules, we should add padding between
the members if necessary.

Bug: chromium:1193170
Change-Id: I75bc3538a66fc0c2a9c9ebf15fddeaab94d7a949
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2790518Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>