AX: Calendar Picker: Support of input[type=month].

The DayCell for the first day of the month rpresents the selected month.  This
might be tricky, but implementation cost is very low.

BUG=123896

Review URL: https://codereview.chromium.org/575353003

git-svn-id: svn://svn.chromium.org/blink/trunk@182222 bbb929c8-8fbe-4397-9dbb-9b2b20218538