Move VkYcbcrSamplerHelper's context functionality into VkTestHelper

This should make the YCbCr sampler helper usable in a GM (with a normal Vk context).

Change-Id: I75451f6ca934f7b59c48349c77234856d0946a12
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295766
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed