Add base::test::ScopedTaskEnvironment.

ScopedTaskEnvironment allows usage of these APIs within its scope:
- ThreadTaskRunnerHandle, on the thread where it lives
- base/task_scheduler/post_task.h, on any thread

Tests that need either of these APIs should instantiate a
ScopedTaskEnvironment.

Design doc
https://docs.google.com/document/d/1QabRo8c7D9LsYY3cEcaPQbOCLo8Tu-6VLykYXyl3Pkk/edit

BUG=708584

Review-Url: https://codereview.chromium.org/2796403002
Cr-Commit-Position: refs/heads/master@{#462477}


CrOS-Libchrome-Original-Commit: a1f53b7c3cca3156dcf7cd7a57264d888ea318ef
3 files changed
tree: ce026dbabc1dfb8896c7e592ee8fdfe9aeb8d882
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/