DevTools: add test to verify matched styles for ::content selector

This patch adds test based on the regression issue with styles sidebar
pane not showing styles for ::contet selectors.

BUG=380414

Review URL: https://codereview.chromium.org/316003002

git-svn-id: svn://svn.chromium.org/blink/trunk@175756 bbb929c8-8fbe-4397-9dbb-9b2b20218538