Allow PeerConnection to be garbage collected after close().

* Add m_closed flag and allow gc after close().
* Assert that no calls from the client comes after close().
* Ensure no callbacks from async operation after close() is called, except getStats.
* Call stop() in the destructor.
* Add tests for garbage collected after close(), but not gc if close() is not called.
* Add tests for getStats() called after close().

BUG=373690

Review URL: https://codereview.chromium.org/329093002

git-svn-id: svn://svn.chromium.org/blink/trunk@176349 bbb929c8-8fbe-4397-9dbb-9b2b20218538