Fix failing DCHECKs in JingleSession added in r294653
r294653 added two DCHECKs in JingleSession in CancelChannelCreation()
and destructor. These checks were failing in the client sometimes. This
CL fixes the code that was causing them to fail:
1. The ChannelMultiplexer needs to be destroyed explicitly in the
destructor.
2. ChannelDispatcherBase was calling CancelChannelCreation() even
after the channel is created.
Review URL: https://codereview.chromium.org/
571273003
Cr-Commit-Position: refs/heads/master@{#295138}