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: I443f3370522639e82f2fa0eebe6b206c372f13a4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228137
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
26 files changed