First draft of Dawn backend: clears are working.

First draft of (mostly stubbed-out) GrDawnGpu.
Skeletons of GrDawnCaps, GrDawnGpuCommandBuffer, GrDawnRenderTarget.
First draft of DawnTestContext.
First draft of psuedo-fences for Dawn, implemented with MapReadAsync.

Change-Id: Id009436f4441f26ffbc82d485d7af3a499b3281b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/226857
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
25 files changed