Make NodeList::ownerNode() virtual and rename to virtualOwnerNode()

Make NodeList::ownerNode() virtual to avoid multiple virtual calls to
is*NodeList() in Node::ownerNode() as well as casting.

NodeList::ownerNode() is also renamed to virtualOwnerNode() to avoid
conflict with LiveNodeListBase::ownerNode(). Also, we don't want
LiveNodeList::ownerNode() to be virtual as it is called from hot
code paths.

R=haraken, adamk

Review URL: https://codereview.chromium.org/161083002

git-svn-id: svn://svn.chromium.org/blink/trunk@167061 bbb929c8-8fbe-4397-9dbb-9b2b20218538