Bug 1478776 - Part 5: Define Visual Viewport event handlers. r=botond,Ehsan

As per https://wicg.github.io/visual-viewport/#the-visualviewport-interface.

Differential Revision: https://phabricator.services.mozilla.com/D14042

--HG--
extra : moz-landing-system : lando

dom/base/VisualViewport.h

@@ -28,6 +28,8 @@ class VisualViewport final : public mozilla::DOMEventTargetHelper {
   double Width() const;
   double Height() const;
   double Scale() const;
+  IMPL_EVENT_HANDLER(resize)
+  IMPL_EVENT_HANDLER(scroll)
 
   virtual JSObject* WrapObject(JSContext* aCx,
                                JS::Handle<JSObject*> aGivenProto) override;

dom/webidl/VisualViewport.webidl

@@ -18,4 +18,7 @@ interface VisualViewport : EventTarget {
   readonly attribute double height;
 
   readonly attribute double scale;
+
+  attribute EventHandler onresize;
+  attribute EventHandler onscroll;
 };