Build flags to enable Vulkan backend and some small updates.

BUG=angleproject:2314

Change-Id: Ia99cc5a4b326a4cab115652cb12b3fb61033eeee
Reviewed-on: https://chromium-review.googlesource.com/909723
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>