Add 'GetAllocationSize' to HeapInterface.

Currently no heaps support this feature. It will be added to the existing heaps one by one in later CLs.

BUG=
R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/131680043

git-svn-id: http://sawbuck.googlecode.com/svn/trunk@2282 15e8cca8-e42c-11de-a347-f34a4f72eb7d