DevTools: Use stylesheet to fill workspace even if they don't have a sourceURL.

Now that we explicitly load the content for CSS style sheets we could solve the issue with empty url() in a css property value.

BUG=84605
NOTRY=true

Review URL: https://codereview.chromium.org/432283002

git-svn-id: svn://svn.chromium.org/blink/trunk@179563 bbb929c8-8fbe-4397-9dbb-9b2b20218538