Prepare Node::dumpStatistics() for oilpan

Bonus: + fix a compile error from AttributeCollection refactoring.

NOTRY because DUMP_NODE_STATISTICS is never enabled on any of the buildbots.
NOTRY=true
BUG=340522

Review URL: https://codereview.chromium.org/372973002

git-svn-id: svn://svn.chromium.org/blink/trunk@177644 bbb929c8-8fbe-4397-9dbb-9b2b20218538