Mac: handle client auth notifications while selector is waiting to be shown.

The observer is only connected after showSheetForWindow:. Connect it as soon as
we have a ConstrainedWindowMac and call CloseWebContentsModalDialog instead of
directly calling closeSheetWithAnimation: so it is aware of modal dialog
queuing.

BUG=54159
TEST=Load a page containing
     <img src="https://FOO/img1.png"><img src="https://FOO/img2.png">
     where FOO is a server for which you have a client certificate.
     There should be one prompt, not two.
     Follow steps in http://crbug.com/140558#c5 for a test case.

Review URL: https://codereview.chromium.org/99133003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239353 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239353