Add autofill preview support for <month> input fields

Add condition handling for suggestedValue case in updateview() and preview functions.
Updated test cases accordingly.

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

Review URL: https://codereview.chromium.org/130213005

git-svn-id: svn://svn.chromium.org/blink/trunk@167933 bbb929c8-8fbe-4397-9dbb-9b2b20218538