Add ScopedTaskEnvironment::GetMainThreadTaskRunner().

This will avoid the need to store the main thread TaskRunner in
a member variable in tests that need to post to the main thread
from other threads (e.g.
https://chromium.googlesource.com/chromium/src/+/134190a648fe48fed0486cf8889569863e4a544c/media/capture/video/video_capture_device_unittest.cc#109)

BUG=708584

Review-Url: https://codereview.chromium.org/2885783002
Cr-Commit-Position: refs/heads/master@{#472111}


CrOS-Libchrome-Original-Commit: 812f416e1368152c31893028eeee7e85bd25e827
2 files changed
tree: 609cd3927172542928fef2046101cab3397eebea
  1. base/
  2. build/
  3. components/
  4. dbus/
  5. device/
  6. ipc/
  7. mojo/
  8. testing/
  9. third_party/
  10. ui/