Vulkan: Allow Vulkan secondary CBs to count their commands This is used by render pass related code to determine if some command has been issued since a previous event. With ANGLE SecondaryCommandBuffer, the memory pointer where the commands are being written to are used for this purpose. For Vulkan secondary command buffers the code simply increments a counter. Bug: angleproject:6100 Change-Id: I85320f8453bd3325793df85aabde3d42b1eeb22a Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3167214 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>