Previously ConnectionToHost was responsible for monitoring state of
the signaling connection and creation of JingleSessionManager. Because
of this it was harder to test and the corresponding logic would need
to be duplicated between all implementations.
Now ChromotingClient monitors state of the signaling connection,
creates JingleSessionManager with a Session and then passes the Session
object to ConnectionToHost.

Review URL: https://codereview.chromium.org/1520323007

Cr-Commit-Position: refs/heads/master@{#365854}