Cleanup ZebraBlockHeap.

This removes some things from the heap and makes them members of the heap itself.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/133160043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2277 15e8cca8-e42c-11de-a347-f34a4f72eb7d