Introduce HeapSnapshotItem interface

The interface describes common part of nodes as well as containment and retainment edges. It used to iterate over the elements and serialize them.

BUG=None

Review URL: https://codereview.chromium.org/209653003

git-svn-id: svn://svn.chromium.org/blink/trunk@169845 bbb929c8-8fbe-4397-9dbb-9b2b20218538