Fix side-effect in DCHECK that breaks another DCHECK in JingleSession.

Fix side-effect in DCHECK that breaks another DCHECK in JingleSession.

DCHECK(!channels_[name]) adds NULL channel with |name| into the
map, which was breaking DCHECK in JingleSession destructor. Replaced
with another DCHECK that doesn't have side-effects. Added unittest
to catch this problem.
Also removed no-op line from ~JingleSession().

Committed: https://crrev.com/8668e3f025c0acc5c77dfb54755835f5f91a5270
Cr-Commit-Position: refs/heads/master@{#296224}

[Sergey Ulanov, 2014-09-23 00:59:10]

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

[Wez, 2014-09-23 16:28:03]

The CQ bit was checked by wez@chromium.org

[Wez, 2014-09-23 16:28:04]

lgtm

[commit-bot: I haz the power, 2014-09-23 16:28:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/594743002/1

[commit-bot: I haz the power, 2014-09-23 20:45:00]

Committed patchset #1 (id:1) as c020f8d8fb6fb149b1efa9f580d70fe4ea4f3682

[commit-bot: I haz the power, 2014-09-23 20:45:52]

Patchset 1 (id:??) landed as
https://crrev.com/8668e3f025c0acc5c77dfb54755835f5f91a5270
Cr-Commit-Position: refs/heads/master@{#296224}