[rAc] [OSX] Set a minimum height for the Autofill dialog's main contents.

BUG=318535
TEST=Shrink Chrome browser window to be as short as possible, ensure that the
     rAc dialog is still usable.
R=groby@chromium.org

Review URL: https://codereview.chromium.org/109453002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239392 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239392