Issue #595 - Implement window.event

This MSIE extension is still technically part of the standard*, although its use is discouraged. This API will also likely never go away based on some comments at this issue on MDN content**.

Note that this uses a different approach for getting the inner window.

* https://dom.spec.whatwg.org/#interface-window-extensions
** https://github.com/mdn/content/issues/21848

Spec PR: https://github.com/whatwg/dom/pull/407
Spec discussion: https://github.com/whatwg/dom/issues/334

Partially based on https://bugzilla.mozilla.org/show_bug.cgi?id=218415

dom/base/nsGlobalWindow.cpp

@@ -909,7 +909,8 @@ nsPIDOMWindow<T>::nsPIDOMWindow(nsPIDOMWindowOuter *aOuterWindow)
   mOuterWindow(aOuterWindow),
   // Make sure no actual window ends up with mWindowID == 0
   mWindowID(NextWindowID()), mHasNotifiedGlobalCreated(false),
-  mMarkedCCGeneration(0), mServiceWorkersTestingEnabled(false)
+  mMarkedCCGeneration(0), mServiceWorkersTestingEnabled(false),
+  mEvent(nullptr)
  {}
 
 template<class T>
@@ -5245,6 +5246,16 @@ nsGlobalWindow::SetOpener(JSContext* aCx, JS::Handle<JS::Value> aOpener,
   SetOpenerWindow(outer, false);
 }
 
+void
+nsGlobalWindow::GetEvent(JSContext* aCx, JS::MutableHandle<JS::Value> aRetval)
+{
+  if (mEvent) {
+    Unused << nsContentUtils::WrapNative(aCx, mEvent, aRetval);
+  } else {
+    aRetval.setUndefined();
+  }
+}
+
 void
 nsGlobalWindow::GetStatusOuter(nsAString& aStatus)
 {