There was an incorrect assumption about how ThreadChecker worked which lead
to several classes not having the d'tor guarantees they expected.  The
assumption was that a ThreadChecker would DCHECK if it was not destroyed on
the same thread it was bound to.  This is not correct so I am adding DCHECKs
to the d'tors of the remoting classes which were missed.

BUG=715633

Review-Url: https://codereview.chromium.org/2890833002
Cr-Commit-Position: refs/heads/master@{#472507}