Fix bugs in partitionDumpStats so that numbers are more accurate.

Also enhance the details reported a little.

R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/27055002

git-svn-id: svn://svn.chromium.org/blink/trunk@159479 bbb929c8-8fbe-4397-9dbb-9b2b20218538