Switch to Dawn's version of dawn_generator.gni. This depends on having
a file called build_overrides/dawn.gni. However, this will also enable
us to use the upstream Dawn BUILD.gn files more easily in the future.
This required adding it to compile.isolate, so the bots can pick it up.
Keeping up with Dawn:
Rename TextureFormat enums.
Rename dawn::BufferUsageBit::TransferDst -> CopyDst.
Removal of GLAD dependency.
SPIRV-Tools update.
Change-Id: Idcd5d1035ed106485dd2503b829e3c3b57a5688b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228568
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
