Add autofill preview support for <month> input fields

Update preview functions to include case for <month> field.
Update tests accordingly.

BUG=335212
R=isherman@chromium.org, jhawkins@chromium.org, tkent@chromium.org

Review URL: https://codereview.chromium.org/140903004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254464 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254464