This is an optimization for SkiaRenderer

Hi, I am willing to fix this.

awesome!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b22ba4aa82126732d2c86de73146607515c7f2cb

commit b22ba4aa82126732d2c86de73146607515c7f2cb
Author: Xu Xing <xing.xu@intel.com>
Date: Fri Nov 02 00:42:34 2018

Make flush optional in SkiaHelper::ApplyImageFilter

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/1312298
Reviewed-by: Robert Kroeger <rjkroege@chromium.org>
Commit-Queue: Xing Xu <xing.xu@intel.com>
Cr-Commit-Position: refs/heads/master@{#604775}
[modify] https://crrev.com/b22ba4aa82126732d2c86de73146607515c7f2cb/components/viz/common/skia_helper.cc
[modify] https://crrev.com/b22ba4aa82126732d2c86de73146607515c7f2cb/components/viz/common/skia_helper.h
[modify] https://crrev.com/b22ba4aa82126732d2c86de73146607515c7f2cb/components/viz/service/display/gl_renderer.cc
[modify] https://crrev.com/b22ba4aa82126732d2c86de73146607515c7f2cb/components/viz/service/display/skia_renderer.cc