GitLab

Flush is necessary for GLRenderer but not SkiaRenderer. So make it optional.

Bug: 897838
Change-Id: I96df913b7afd1fb6bf84314e3bd7ee0ed25ac497
Reviewed-on: https://chromium-review.googlesource.com/c/1312298Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Xing Xu <xing.xu@intel.com>
Cr-Commit-Position: refs/heads/master@{#604775}