Modify ZebraBlockHeap to accept a heap to use for internal allocations.

This also adds an 'internal heap' to the BlockHeapManager. With this change
the ZebraBlockHeap no reports internal memory use directly to the shadow
memory.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/132410043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2289 15e8cca8-e42c-11de-a347-f34a4f72eb7d