Currently all DisplayVk implementations depend on VK_KHR_swapchain and
VK_KHR_surface extensions. When running Chromium on Fuchsia these
extensions are never used (content is shown on the screen using
ImagePipe API without dependency on swapchain). ANGLE still depended
on these extensions for DisplayVkFuchsia.
This CL adds DisplayVkNull, which allows to run ANGLE without dependency
on swapchain. It's usable only offscreen and cannot present content on
a surface.

Bug: chromium:1203879
Change-Id: I5cadcdf46ed1cfb5ebb3cb69dbfef063e9e2b826
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3012368Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>