This is necessary so we can initialize a "null" BufferHelper in
RendererVk which does not have access to the ContextVk. This in
turn will allow us to use a single global "null" Buffer instead
of instantiating them all over ANGLE.

Also removes a TODO that was sticking in the code.

Bug: angleproject:2162
Bug: chromium:1086532
Change-Id: Ica48d5b886e885ebfe0f8e3abfbe8169a8eaa5b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2219139
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>