Oilpan: Replace RefPtrs to Node and its subclasses in core/rendering with Oilpan transition types.

BUG=357163

Review URL: https://codereview.chromium.org/318583002

git-svn-id: svn://svn.chromium.org/blink/trunk@175457 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Oilpan: Make ActiveParserSession::m_document traceable.

ActiveParserSession, NestingLevelIncrementer, and PumpSession are allocatable
only on stack. We can make m_document Member<> without trace().

BUG=357163

Review URL: https://codereview.chromium.org/313033003

git-svn-id: svn://svn.chromium.org/blink/trunk@175458 bbb929c8-8fbe-4397-9dbb-9b2b20218538