AX: Calendar Picker: Add AX labels to MonthPopupButton and CalendarNavigationButtons.

This CL adds no new tests. Will add tests after a Chromium change for
string resource.

BUG=123896

Review URL: https://codereview.chromium.org/552163002

git-svn-id: svn://svn.chromium.org/blink/trunk@181617 bbb929c8-8fbe-4397-9dbb-9b2b20218538