Mac bookmark mgr: Fix crash after closing incognito window.
BUG=32594
TEST=none (see steps to reproduce)
Review URL: http://codereview.chromium.org/554034

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@36772 0039d316-1c4b-4281-b951-d872f2087c98