Implement flush finish proc support for d3d backend.

This also adds a GPUTEST_FOR_D3D_CONTEXT macro to help with debugging
tests.

Change-Id: I72db01d148755c3bbbbb4d948d441a31dcf9482b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/297717
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
8 files changed