Re: [webcomponents] Define the behavior of '::slotted' (the former name is '::content'). (#331)

Here's the example for @rniwa
http://jsbin.com/yequpu/edit?html,js,output

```html
<div id="host">
  :shadow-root
    <style>::slotted(span) { color: green; background-color: yellow; }</style>
    <div id="host2">
      :shadow-root
        <style>::slotted(span) { color: red; }</style>
        <slot name="slot2"></slot>
      <slot name="slot1" slot="slot2"></slot>
    </div>
  <span slot="slot1">Hello, Shadow DOM</span>
</div>
```

In current Blink's implementation, first `{ color: green; background-color: yellow; }` is applied,
then `{ color: red; }` is applied on top of it -> result is `{ color: red, background-color: yellow; }`.


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https://github.com/w3c/webcomponents/issues/331#issuecomment-193169546