Vulkan: Refactor CommandBuffer helper class. This makes the base vk::CommandBuffer class as close to the Vulkan API as possible. It moves the extra functionality and state tracking to a CommandBufferAndState helper class. Also no longer stores a reference to a CommandPool in a CommandBuffer. Eventually we won't need to free CommandBuffers explicitly, since they can be freed en-masse by deallocating the CommandPool on a flush. Bug: angleproject:2200 Change-Id: I5095fe89e8da935ff8273b1a402ccfd6a3ffe0df Reviewed-on: https://chromium-review.googlesource.com/778184 Reviewed-by: Frank Henigman <fjhenigman@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>