Revert "First draft of Dawn backend: clears are working."

This reverts commit 6cebea42a8cd49e8cd405bfdd59ddee081bf9d97.

Reason for revert: breaking google3 roll

Original change's description:
> 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>

TBR=egdaniel@google.com,jvanverth@google.com,bsalomon@google.com,senorblanco@chromium.org

Change-Id: Ie494b5a403e8537c6539551533ae8b9156e90a61
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/228120
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
25 files changed