Oilpan: update remaining Document uses of RefPtr<Node>s.

Made hoverNode()/setHoveNode() private accessors while converting over
remaining PassRefPtr<Node>/RefPtr<Node> uses to transition types.

R=tkent@chromium.org
BUG=357163

Review URL: https://codereview.chromium.org/311053003

git-svn-id: svn://svn.chromium.org/blink/trunk@175470 bbb929c8-8fbe-4397-9dbb-9b2b20218538