Oilpan: Prepare to move XPath::NodeSet, XPath::ValueData, and XPath::Value.

Note:
- XPath::Value is a part object of XPathResult.
- XPath::NodeSet is on-heap, a part object of XPath::ValueData and
XPathResult, is copyable.

This is a preparation to remove RefPtr<Attr> and RefPtr<Node>.

BUG=357163

Review URL: https://codereview.chromium.org/275053005

git-svn-id: svn://svn.chromium.org/blink/trunk@174061 bbb929c8-8fbe-4397-9dbb-9b2b20218538