Oilpan: Replace RefPtrs to Node and its subclasses in core/html with Oilpan transitin types.

This CL also declares VTTTreeBuilder as STACK_ALLOCATED and makes m_currentNode
and m_document traceable.

BUG=357163

Review URL: https://codereview.chromium.org/315843004

git-svn-id: svn://svn.chromium.org/blink/trunk@175467 bbb929c8-8fbe-4397-9dbb-9b2b20218538