Implemented FenceNVVk, based on the existing vk::SyncHelper class.

Bug: angleproject:4295
Change-Id: I3f44a66e27ce3bd24461894dae4757b25321a6a4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2013880
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>