DevTools: [CSS] remove method getStyleSheet from protocol.json

This patch removes method getStyleSheet from protocol.json.
The only client of the method was Audits panel, which now uses front-end
tokenizer to build style sheed object model.

Drive-by: annotations added to AuditRules.js

BUG=341506

Review URL: https://codereview.chromium.org/163973002

git-svn-id: svn://svn.chromium.org/blink/trunk@167346 bbb929c8-8fbe-4397-9dbb-9b2b20218538