Add more general flush call to GrContext.

This is to match the current features of SkSurface::flush and to prepare
for adding additional features to flush.

Bug: skia:8802
Change-Id: I5d68272e1277b416af357e6ffaf426841ceda943
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/207301
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
31 files changed