Replace CursorList with RefVector<CursorData>.

We recently added a template in wtf/ for any case where we want to
store a Vector in a RefPtr. The list of CursorData objects should be
implemented by re-using that template, rather than with its own special
class which does exactly the same thing.

I added RefVector::at, because it's used by existing CursorList-code.
(See CSSComputedStyleDeclaration:1875).

Review URL: https://codereview.chromium.org/291863003

git-svn-id: svn://svn.chromium.org/blink/trunk@174367 bbb929c8-8fbe-4397-9dbb-9b2b20218538