Documenting that the observer can safely be destroyed after Close has
been called, because it ensures no more callbacks will be invoked. Just
like in JavaScript land, where no more events will be fired after
"close" is called.
This is already covered by unit tests.
BUG=webrtc:7491
NOTRY=True
TBR=pthatcher@webrtc.org
Review-Url: https://codereview.webrtc.org/2834543005
Cr-Commit-Position: refs/heads/master@{#17798}
d07061c9f3