Take references instead of pointers in TreeScope constructor

Also store a Node& in m_rootNode. This allows us to avoid some useless
null checks and makes other code that was already assuming non-nullity
look safer.

R=ch.dumez@samsung.com

Review URL: https://codereview.chromium.org/143953013

git-svn-id: svn://svn.chromium.org/blink/trunk@165597 bbb929c8-8fbe-4397-9dbb-9b2b20218538