interface: use Web IDL's new-ish interface mixins concept (#1176)

WebIDL recently introduced dedicated syntax for mixins [1]. This
replaces the existing [NoInterfaceObject] and "implements" syntax with
"interface mixin" and "includes" in the appropriate places.

Fixes: #1140
Test: web-platform-tests/wpt#8770

webdriver-spec.html

@@ -740,12 +740,11 @@ <h2 data-dfn-for="NavigatorAutomationInformation">Interface</h2>
  the user agent must support <dfn>NavigatorAutomationInformation</dfn> interface:
 
  <pre class=idl>
- Navigator implements NavigatorAutomationInformation;
+ Navigator includes NavigatorAutomationInformation;
 </pre>
 
 <pre class=idl>
- [NoInterfaceObject, Exposed=(Window)]
- interface NavigatorAutomationInformation {
+ interface mixin NavigatorAutomationInformation {
  readonly attribute boolean webdriver; // always returns true
  };
 </pre>