This change removes the constructor parameter of ClientSession, which is used to
initialize DataChannelManager. Instead, a public function
ClientSession::RegisterDataChannelManagerCallback() has been added to achieve
the goal.

The motion of this change is to avoid the base::Bind() to be kept in a secondary
place in ChromotingHost other than ClientSession. And I think it's also
reasonable to public the Register callback function from ClientSession. So we
do not need to write all registering logic in ClientSession.

BUG=650926

Review-Url: https://codereview.chromium.org/2967793002
Cr-Commit-Position: refs/heads/master@{#485482}