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}
2 files changed