DevTools: [Audits] Fix the way the category counter is updated

Instead of tracking clicks, the checked categories are counted
upon each checkbox change.

R=pfeldman, vsevik
BUG=263328

Review URL: https://chromiumcodereview.appspot.com/21239002

git-svn-id: svn://svn.chromium.org/blink/trunk@155192 bbb929c8-8fbe-4397-9dbb-9b2b20218538