This switches from using resource barriers for buffers to using global
barriers. This matches the general advised best practice. It also
allows us to combine multiple barriers into one. On a draw we might
combine all the vertex and index barriers into a single barrier call.

We implement this using a bit of extra state tracking in BufferHelper.

Bug: angleproject:2828
Change-Id: I196b368804ff50e60d085687a643e5566ba1c5b6
Reviewed-on: https://chromium-review.googlesource.com/c/1309977
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>