Fix a crash when we try to close a js dialog that wasn't shown.

      The dialog has been queued, but it hasn't been shown because a
different dialog is already showing.  We try to close the dialog
because a page navigation has occurred.

BUG=47056
TEST=None

Review URL: http://codereview.chromium.org/2803017

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@50561 0039d316-1c4b-4281-b951-d872f2087c98