AX: Make the calendar role "grid".

Rows have role="row", and cells have role="girdcell".

We apply aria-activedescendant to the calendar because the calendar cell is not
focusable. Thus, each of cells needs to have id=.  Also, cells should have
descriptive text as aria-label.

- getLocale() needs to replace _ with - because Intl.DateTimeFormat doesn't
  accept _.

- This CL doesn't improve the calendar picker for type=month and type=week, and
  won't be harmful for them.

BUG=123896

Review URL: https://codereview.chromium.org/536903002

git-svn-id: svn://svn.chromium.org/blink/trunk@181462 bbb929c8-8fbe-4397-9dbb-9b2b20218538