Add Dawn repo to DEPS.
Add skia_use_dawn flag to BUILD.gn (default false).
Add Metal.framework for Mac when building for Dawn/Metal.
Add d3d12, dxgi, d3dcompiler libs when building for Dawn/D3D12.
Disable SPIRV validation when building for Dawn.
Add BUILD.gn for dawn and dawn/third_party libraries.
(Note that I had to split the declare_args() section of BUILD.gn
into two, so that I could refer to skia_use_dawn in skia_enable_spirv_validation.
If there's a better way to do this, I'm open to suggestions.)
Change-Id: I9bdaf990694f6b8bcce6b77c1a8b1620a7396e34
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/225137
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
