Stop returning an Option from Window::browsing_context.

A Window always has a WindowProxy; the only reason it's wrapped in a nullable
field is the order in which those objects are created.

components/script/dom/bindings/utils.rs

@@ -395,7 +395,7 @@ pub unsafe extern "C" fn outerize_global(_cx: *mut JSContext, obj: HandleObject)
     debug!("outerizing");
     let win = root_from_handleobject::<window::Window>(obj).unwrap();
     let context = win.browsing_context();
-    context.as_ref().unwrap().window_proxy()
+    context.window_proxy()
 }
 
 /// Deletes the property `id` from `object`.