In preparation of running DirectTransport on a TaskQueue in unit-tests, change the thread-checkers to sequence-checkers. This is necessary for Mac and iOS, where the TaskQueue is guaranteed to run sequentially, but not guaranteed to do so on only one thread.
TODO: Add the relevant BUGs.
BUG=None
Review-Url: https://codereview.webrtc.org/2999973002
Cr-Commit-Position: refs/heads/master@{#19404}