Calendar Picker: Make month popup cells non-focusable.

We used <button> for cells, and we had unexpected focus behaviors. This CL
changes <button> to <div>.
This is a preparation to support accessibility in the month popup.

BUG=123896

Review URL: https://codereview.chromium.org/556323003

git-svn-id: svn://svn.chromium.org/blink/trunk@181781 bbb929c8-8fbe-4397-9dbb-9b2b20218538