Print out how many AllocationMementos were found during mark-sweep.

Moreover use the right memory boundary for AllocationMemento lookup during gc.

BUG=
R=mvstanton@chromium.org

Review URL: https://codereview.chromium.org/25655004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17069 ce2b1a6d-e550-0410-aec6-3dcde31c8c00