Adds a counter for the secondary command buffers (non-RenderPass).
We'll use this in an upcoming test that validates that ANGLE only
issues a single barrier (CB) in some buffer read/write scenarios.

Also adds a PerfCounters struct.

Bug: angleproject:4429
Change-Id: Iaf75ca89da3d02753897cb4066e2c56db497417e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2334090Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>