Oilpan: Make the Node hierarchy RefCountedGarbageCollected instead of TreeShared.

The entire tree structure is now traced. If one node in the tree is
alive the entire tree is kept alive. This makes the Node hierarchy
independent of reference counts and it is therefore possible to use
Members instead of RefPtrs to Nodes.

This change is very likely to introduce new Oilpan leaks because
any RefPtr from a Node to another Node in the same tree will leak
the entire tree. Once this change lands we should eliminate RefPtrs
to Nodes to avoid these leaks.

BUG=357163

Review URL: https://codereview.chromium.org/262093006

git-svn-id: svn://svn.chromium.org/blink/trunk@173378 bbb929c8-8fbe-4397-9dbb-9b2b20218538