AX: Calendar Picker: Apply aria-activedescendant to YearListView.

Month buttons should have role=gridcell, and YearListCell should not. So this CL
moves role=gridcell from ListCell to DayCell.
Month buttons should have their own IDs, and readable month label.

BUG=123896

Review URL: https://codereview.chromium.org/564053002

git-svn-id: svn://svn.chromium.org/blink/trunk@181887 bbb929c8-8fbe-4397-9dbb-9b2b20218538