It turns out it won't be easy to just use the RenderTargetVk and
CommandBuffer with the Command Graph refactor. Better to use the
FramebufferVk directly.

Also cleans up a few incidental style issues.

Bug: angleproject:2539
Change-Id: I73c2767072a84e9e5042361c7bdf4e9aef0f8ac7
Reviewed-on: https://chromium-review.googlesource.com/1066471Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>