DevTools: Dispatch StyleSheetChanged event regardless of majorChange value

R=pfeldman, vsevik
BUG=312273

Review URL: https://codereview.chromium.org/48853002

git-svn-id: svn://svn.chromium.org/blink/trunk@160942 bbb929c8-8fbe-4397-9dbb-9b2b20218538