shim styling: need to support ^ and ^^ when they are defined outside of <polymer-element>

Currently shim styling processes ^ and ^^ when they are defined inside of &lt;polymer-element&gt;.  But when they are defined outside of &lt;polymer-element&gt; shim styling doesn't process them. e.g.

``` html
<head>
  <style>
    body ^^ x-foo {
      ...
    }
  </style>
</head>
```

We need to provide a tool and/or make html import to process stylesheet that contains ^ and ^.
