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

This is a reland of 3b7a12bd66de0d55792215372708083ac8906617

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>

Bug: chromium:1171141
Bug: chromium:1171188
Change-Id: I535c9e0252b73c5277b0257ea5cdcac92de53e70
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369877
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Adlai Holler <adlai@google.com>
3 files changed