DevTools: [SSP] add heuristic for css property value parsing

This patch adds a heuristic for parsing CSS url(..) statements.

BUG=414897
R=vsevik

Review URL: https://codereview.chromium.org/645163004

git-svn-id: svn://svn.chromium.org/blink/trunk@184016 bbb929c8-8fbe-4397-9dbb-9b2b20218538