Add flag to map buffer flags in GrCaps to indicate whether mapping a
GrGpuBuffer for reading is synchronous or not.

Update transfer-from test to not call GrGpu->finishFlush() when maps
are synchronous.

Bug: skia:8962
Change-Id: I5eb003081725e5199157473faf7f916a65408dc8
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208509
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
4 files changed