This is not part of the public API of Dawn and will be unnecessary once
Dawn move to "surface based swapchains". DawnD3D12WindowContext didn't
need the include so all that's needed is copying the helper in
DawnMTLWindowContext.
Change-Id: I8d0682b6b0801dc311ad4c7d73d4c07e575ecf4e
Bug: chromium:1064305
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282496
Reviewed-by: Zhenyao Mo <zmo@google.com>
Reviewed-by: Stephen White <senorblanco@chromium.org>