Force flush in clear benchmarks

Bug: skia:
Change-Id: I9d373dbcf78c6fe52f74deb37d8e08595d3a7c28
Reviewed-on: https://skia-review.googlesource.com/c/184064
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
1 file changed