Surprisingly, this is not a tab-modal sheet on linux and windows, so it's window-modal on os x as well for now.

BUG=23526
TEST=Go to http://www.cs.unc.edu/~jbs/resources/perl/perl-cgi/programs/form1-POST.html , click "Do it!", hit reload. Window sheet should come up. Hitting cancel should cancel the navigation (and hitting reload again should bring up sheet again). Hitting "Resend" should trigger reload.

Review URL: http://codereview.chromium.org/327009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@30547 0039d316-1c4b-4281-b951-d872f2087c98