feat: implement CSS Coverage (#1714)

This patch adds two new methods to the `page.coverage` namespace:
- `page.coverage.startCSSCoverage()` - to initiate css coverage
- `page.coverage.stopCSSCoverage()` - to stop css coverage

The coverage format is consistent with the JavaScript coverage.

utils/doclint/check_public_api/index.js

@@ -20,6 +20,7 @@ const Documentation = require('./Documentation');
 const Message = require('../Message');
 
 const EXCLUDE_CLASSES = new Set([
+  'CSSCoverage',
   'Connection',
   'Downloader',
   'EmulationManager',