SequencedTaskChecker is a helper class used to help verify that some methods
of a class are called on the same task queue or thread. A
SequencedTaskChecker is bound to a a task queue if the object is
created on a task queue, or a thread otherwise.
BUG=webrtc:5687
Review-Url: https://codereview.webrtc.org/2125113003
Cr-Commit-Position: refs/heads/master@{#13452}
