DevTools: [SSP] Implement adding new rule in user stylesheet

This patch implements an "add new rule" functionality to add a style rule in a user stylesheet. It is also possible to
add rule inside media query rules.

BUG=402461

Review URL: https://codereview.chromium.org/441873010

git-svn-id: svn://svn.chromium.org/blink/trunk@180164 bbb929c8-8fbe-4397-9dbb-9b2b20218538