Revert "Flush rather than copy pixels in GrSurfaceContext::writePixels."

This reverts commit 3b7a12bd66de0d55792215372708083ac8906617.

Reason for revert: metal command buffer assert?

Original change's description:
> Flush rather than copy pixels in GrSurfaceContext::writePixels.
>
> Speculative fix for:
>
> Bug: chromium:1171141
> Bug: chromium:1171188
> Change-Id: Ic9762f17652172e620c6bb59aa89e3bb6bc97659
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369318
> Reviewed-by: Adlai Holler <adlai@google.com>
> Commit-Queue: Brian Salomon <bsalomon@google.com>

TBR=bsalomon@google.com,adlai@google.com

Change-Id: I5db0cfabb1396db3d4fe9f9c6809d5de72d6572f
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: chromium:1171141
Bug: chromium:1171188
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369476
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed