Let SkTaskGroup work synchronously if no one created an SkTaskGroup::Enabler.

Tested by running DM with its SkTaskGroup::Enabler commented out.  Slow, but completed correctly.

BUG=skia:
R=reed@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/556223003
1 file changed