Oilpan: chrome://tracing info wrong

1. change
    json->setInteger("pageCount", info.freeSize);
to
    json->setInteger("pageCount", info.pageCount);

R=ager@chromium.org, oilpan-reviews@chromium.org
BUG=408497

Review URL: https://codereview.chromium.org/515123002

git-svn-id: svn://svn.chromium.org/blink/trunk@181073 bbb929c8-8fbe-4397-9dbb-9b2b20218538