Uses an unordered_map in the CommandBufferHelper to track buffer
reads and writes. Buffer barriers are tracked specially in the
CommandBufferHelper class as a barrier we execute immediately when
we execute the commands into the primary. So when we run into an
incompatible buffer access we must start a new command buffer.

The rules for an incompatible access are:

 - when we are reading a buffer, any prior write in the same command
   buffer is incompatible.
 - when we are writing a buffer, any prior read or write in the same
   command buffer is incopatible.

Also adds a regression test using a new performance counter.

Bug: angleproject:4429
Change-Id: I393a4ed87314f955eb998940b877ba76ea15a7b8
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2334091Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>