[rAc OSX] Prevent interaction with the main contents of the dialog when the overlay is visible.

BUG=321309,321328
TEST=(see bugs)
R=groby@chromium.org

Review URL: https://codereview.chromium.org/82863003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236770 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236770