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

Note:
- XPath::Value is a part object of XPathResult.
- XPath::NodeSet was a part object of XPathResult or XPath::ValueData, and
  NodeSet is also used as an independent object. This CL makes NodeSet not a
  part object because using GarbageCollected object as a part object is
  dangerous.

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

BUG=357163,373814
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/291763002

git-svn-id: svn://svn.chromium.org/blink/trunk@174239 bbb929c8-8fbe-4397-9dbb-9b2b20218538