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
Don't share the Custom Element registration context with templates.

Elements in templates are meant to be inert. (Scripts don't run, for
example.) So that Custom Elements aren't "live" (upgraded, running
callbacks) in templates, stop sharing the main document's registration
context with the template document.

Authors that want Custom Element definitions to be active in templates
can register definitions with the template document directly.

This is described here:
<https://www.w3.org/Bugs/Public/show_bug.cgi?id=23839>

BUG=234509

Review URL: https://codereview.chromium.org/72773003

git-svn-id: svn://svn.chromium.org/blink/trunk@162301 bbb929c8-8fbe-4397-9dbb-9b2b20218538