Oilpan: Remove manual ref/deref of Nodes  in XMLDocumentParser.

Objects in XMLDocumentParser::m_currentNodeStack are correctly traces in
Oilpan. We don't need ref/deref.

Also, this CL replace one instance of Vector<Element*> with
HeapVector<Member<Elemenet>>.

BUG=357163

Review URL: https://codereview.chromium.org/320773002

git-svn-id: svn://svn.chromium.org/blink/trunk@175763 bbb929c8-8fbe-4397-9dbb-9b2b20218538