Add histogram for total heap fragmentation, don't report fragmentation for new space, and report fragmentation, not usage

BUG=none
TEST=none

Review URL: https://chromiumcodereview.appspot.com/10778009
Patch from Jochen Eisinger <jochen@chromium.org>.

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12099 ce2b1a6d-e550-0410-aec6-3dcde31c8c00