Add GetMinimumSize() for Borders, and make LabelButton auto-size to at least as
large as its border requires.

BUG=311269
TEST=none
R=msw@chromium.org

Review URL: https://codereview.chromium.org/82483003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@236912 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 236912