DevTools: Handle stylesheets sharing the same frameId and URL properly

R=vsevik@chromium.org
BUG=260267

Review URL: https://codereview.chromium.org/19205002

git-svn-id: svn://svn.chromium.org/blink/trunk@154289 bbb929c8-8fbe-4397-9dbb-9b2b20218538