[Mac] Log stack trace for OnChannelError() in bug 97285.
The hypothesis in comment #67 is that OnChannelError() is being called
to invalidate a channel, and the invalid channel is later being vended
from the cache. This will verify if OnChannelError() was previously
called on the channel, and also provide a backtrace to determine what
the context was.
BUG=97285
TEST=crash server
Review URL: https://chromiumcodereview.appspot.com/
10544038
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@140921 0039d316-1c4b-4281-b951-d872f2087c98