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>
3 files changed