Vulkan: Pass command pool around Preparatory work for re-enabling support for Vulkan secondary command buffers. The command pool being passed around is not yet used. Additionally, the work to allocate, initialize, reset and recycle command buffers has been refactored to a helper class for better abstraction. The CommandBufferHelper class now holds a pointer to the pool it was allocated from for clean up. Each ContextVk has its own command pool, where Vulkan secondary command buffers are allocated from. The command pool that CommandProcessor had was unused and is removed by this change. Bug: angleproject:6100 Change-Id: I0d4fa6a76ef06e88779145ffe9c613518a8eb390 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3167213 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>