Replace more off-heap collections of Node* pointers with on-heap collections

This is a trivial CL.

BUG=340522

Review URL: https://codereview.chromium.org/384623002

git-svn-id: svn://svn.chromium.org/blink/trunk@177899 bbb929c8-8fbe-4397-9dbb-9b2b20218538