Fix failing DCHECKs in JingleSession added in r294653

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.

Committed: https://crrev.com/d04beefcfe9d886bd0938ce409b2236482c6c1b8
Cr-Commit-Position: refs/heads/master@{#295138}

[Sergey Ulanov, 2014-09-16 01:35:46]

sergeyu@chromium.org changed reviewers:
+ wez@chromium.org

[Wez, 2014-09-16 19:22:59]

The CQ bit was checked by wez@chromium.org

[Wez, 2014-09-16 19:23:00]

lgtm

[Wez, 2014-09-16 19:23:31]

Can these two issues be easily unit-tested for, to prevent regressions?

[commit-bot: I haz the power, 2014-09-16 19:23:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/571273003/1

[commit-bot: I haz the power, 2014-09-16 20:24:07]

Committed patchset #1 (id:1) as 998c7ce8c1b07821416e98b27a92476013c1d33f

[commit-bot: I haz the power, 2014-09-16 20:25:20]

Patchset 1 (id:??) landed as
https://crrev.com/d04beefcfe9d886bd0938ce409b2236482c6c1b8
Cr-Commit-Position: refs/heads/master@{#295138}