Remove TabContents from TabStripModelObserver::TabDetachedAt.

BUG=107201
TEST=no visible change


Review URL: https://chromiumcodereview.appspot.com/11293205

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167122 0039d316-1c4b-4281-b951-d872f2087c98
  virtual void Observe(int type,
                       const content::NotificationSource& source,
                       const content::NotificationDetails& details) {
    TabContents* tab_to_delete = tab_to_delete_;
    tab_to_delete_ = NULL;
    delete tab_to_delete;
  }

  virtual void Observe(int type,
                       const content::NotificationSource& source,
                       const content::NotificationDetails& details) {
    TabContents* tab_to_delete = tab_to_delete_;
    tab_to_delete_ = NULL;
    delete tab_to_delete;
  }
