DevTools: Make AuditCategory an interface to avoid bogus @extends for ExtensionAuditCategory.

R=aandrey, eustas, vsevik, yurys

Review URL: https://codereview.chromium.org/114793006

git-svn-id: svn://svn.chromium.org/blink/trunk@164329 bbb929c8-8fbe-4397-9dbb-9b2b20218538