Introduce TaskTracker::FlushAsyncForTesting()

This performs a task flush similar to TaskTracker::Flush() except it is
async. It calls the callback when TaskTracker::Flush() would have
returned.

BUG=804930

Change-Id: I428fd822104a3c5c0663a05852d7ecc47680c2ae
Reviewed-on: https://chromium-review.googlesource.com/882384
Commit-Queue: Robert Liao <robliao@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531719}

CrOS-Libchrome-Original-Commit: 089b37fd14d1de4ce71e10842ed1bdb689392b55
3 files changed
tree: f1eca178e5d4ae68e40974f1a25b6122e7ae7b91
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/