Oilpan: Make sure XMLTreeViewer::m_document is traceable.

- Mark XMLTreeViewer STACK_ALLOCATED().
- Make m_document Member<>.

BUG=357163

Review URL: https://codereview.chromium.org/339703004

git-svn-id: svn://svn.chromium.org/blink/trunk@176263 bbb929c8-8fbe-4397-9dbb-9b2b20218538