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/nsPIDOMWindow.h

@@ -194,6 +194,10 @@ protected:
   // we have what it takes to do so.
   void MaybeCreateDoc();
 
+  // The event dispatch code sets and unsets this while keeping
+  // the event object alive.
+  nsIDOMEvent* mEvent;
+
 public:
   inline bool IsLoadingOrRunningTimeout() const;
 
@@ -789,6 +793,17 @@ public:
     return mInnerObjectsFreed;
   }
 
+  // Sets the event for window.event. Does NOT take ownership, so
+  // the caller is responsible for clearing the event before the
+  // event gets deallocated. Pass nullptr to set window.event to
+  // undefined. Returns the previous value.
+  nsIDOMEvent* SetEvent(nsIDOMEvent* aEvent)
+  {
+    nsIDOMEvent* old = mEvent;
+    mEvent = aEvent;
+    return old;
+  }
+
   /**
    * Check whether this window is a secure context.
    */