Add GetAllocationSize support to ZebraBlockHeap.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/135210045

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2288 15e8cca8-e42c-11de-a347-f34a4f72eb7d