Update nanobench and skpbench to use flush API for gpu syncing.

This also allows us to remove all the one off Fence code that we
implemented in all the backend TestContexts

Change-Id: I9ff7ba4690cf3f19a180f51fc510991a112bb62c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/272456
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
24 files changed