Pull CommandBufferHelper class out of ContextVk.h/cpp and move it
to vk_helpers.h/cpp. This is the natural place for it as it's a
helper class. Also, this class is planned to be the interface between
the main and worker threads so moving it to vk_helpers makes it easy
to share between Context and Renderer.

Bug: b/154030403
Change-Id: Ie5eeb864164a3787f800905ae885027834bd1a08
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2182177Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>