Fix partitionDumpStats.

It had bitrotted because no test executes the code path. The tests now also
execute the code path.

(Some follow-up work is apparent, to be taken care of in further CLs, such as
supporting partitionDumpStatsGeneric, moving to a typed API for stronger
testing, and perhaps making the facilities available in Release builds for
support of something like about:tcmalloc).

Review URL: https://codereview.chromium.org/280833004

git-svn-id: svn://svn.chromium.org/blink/trunk@174054 bbb929c8-8fbe-4397-9dbb-9b2b20218538