Reduce GrOnFlushResourceProvider's functionality

The main goal of this CL was to remove GrOnFlushResourceProvider::makeSurfaceDrawContext (which was only being used in the OnFlushCallbackTest) to allow simpler modifications to GrSurfaceDrawContext. It looks like there was a lot more unused stuff here though.

Bug: skia:11837
Change-Id: Idbc4001c58a4e8456bffcf1bac32150f2452fb56
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/430043
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
8 files changed