Remove NOTIFICATION_WEB_CONTENTS_DESTROYED from ContentSettingBubbleGtk.

BUG=170921
TEST=everything still works

Review URL: https://codereview.chromium.org/27190004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228826 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228826