GitLab

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().

Review URL: https://codereview.chromium.org/594743002

Cr-Commit-Position: refs/heads/master@{#296224}