Add a new method to check if an address point to the beginning of an user alloc

This will be useful to check if a block has been allocated without guards.

R=chrisha@chromium.org

BUG=

Review URL: https://codereview.appspot.com/131430043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2275 15e8cca8-e42c-11de-a347-f34a4f72eb7d