Implement Window contexts for D3D12, Metal and Vulkan, as well as
a base class for all of them (DawnWindowContext).
Implement WSI, swap chains and external textures for all backends.
Add Dawn support to Viewer app.
Change-Id: I9368eae8d43594821aa1edd9fd559c8a9ba30066
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228060
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
