This makes the vk::CommandBuffer class closer to an auto-generated
wrapper class.

Bug: angleproject:2828
Change-Id: I91ba03914a60d5612707dd240fcda25b86cf2c46
Reviewed-on: https://chromium-review.googlesource.com/1255505
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>