Set minimal dialog size.
Preferred dialog size now is not minimal.

320 chosen as Help Center box on the help dialog looks good with that width.

BUG=174404
NOTRY=true

Review URL: https://codereview.chromium.org/26992002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228484 0039d316-1c4b-4281-b951-d872f2087c98
WebUI: Fix issue with list selection

This is a regression from r227470

BUG=None
R=tkent@chromium.org

Review URL: https://codereview.chromium.org/27011004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228485 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228485