Enable threaded SkTaskGroup in Viewer

BUG=skia:

Change-Id: I7f467c094fb0bcb983bd86d07cb9bd7be34666b3
Reviewed-on: https://skia-review.googlesource.com/8332
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
1 file changed