[graphite] Remember last snapped task for scratch devices

Change-Id: I3297b584fd48dc7151d4fff54218c1da2bbfcd5a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/835576
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
2 files changed