Fix crash in XmppSignalStrategy.
Fix crash in XmppSignalStrategy.

XmppClient destroys itself when connection is closed. Session objects are destroyed later and may still try to send a message. XmppSignalStrategy didn't handle this case properly.

BUG=100733
TEST=See repro steps in the bug.


Review URL: http://codereview.chromium.org/8344006

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@106177 0039d316-1c4b-4281-b951-d872f2087c98

yitian134/chromium,ropik/chromium,adobe/chromium,ropik/chromium,adobe/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,adobe/chromium,adobe/chromium,ropik/chromium,gavinp/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,ropik/chromium,gavinp/chromium,gavinp/chromium,adobe/chromium,ropik/chromium,adobe/chromium,adobe/chromium,yitian134/chromium,ropik/chromium,ropik/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium,yitian134/chromium