Make nanobench --mpd flush between canvases.

This more closely mirrors Chrome GPU raster.

Bug: skia:8514
Change-Id: I6a038fd393d45dc48808d225b84f4e215e3a79b7
Reviewed-on: https://skia-review.googlesource.com/c/166800
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed