DevTools: Improve performance of WebInspector.Project.forEachUISourceCode

Review URL: https://chromiumcodereview.appspot.com/16874006

git-svn-id: svn://svn.chromium.org/blink/trunk@152885 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Do not report some CSS errors for code valid in other browsers.

R=pfeldman, vsevik
BUG=252105

Review URL: https://chromiumcodereview.appspot.com/17176013

git-svn-id: svn://svn.chromium.org/blink/trunk@152886 bbb929c8-8fbe-4397-9dbb-9b2b20218538