Make second argument to CSSStyleSheet.insertRule() optional again

Make second argument to CSSStyleSheet.insertRule() optional again to restore
backward compatibility. Also add a deprecation warning as passing only one
argument to CSSStyleSheet.insertRule() is incompatible with the standard
specification and with other major browsers.

Making the arguments mandatory was done in r157074 but broke a popular
jQuery plugin.

BUG=319695
R=tkent

Review URL: https://codereview.chromium.org/63883004

git-svn-id: svn://svn.chromium.org/blink/trunk@162300 bbb929c8-8fbe-4397-9dbb-9b2b20218538