add SkTaskGroup::done()

This lets you check if the work's done without blocking.
Seems handy.

Change-Id: Ie27c7b6fe0d01262b6a777abbc18b0de108641c0
Reviewed-on: https://skia-review.googlesource.com/38120
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
2 files changed