Issue #1792 - Part 3: Add ConstructibleEventTarget helper class.

Of course it couldn't be trivial :P
wrapObject is a pure virtual function which prevents an abstract constructor
from being used.
So, we need to extend DOMEventTargetHelper (applicable acronym: DETH :D) with
a wrapObject implementation - in a derivative class is the clearest way to do
this, as opposed to making DETH more complex.

dom/events/ConstructibleEventTarget.h

@@ -0,0 +1,34 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ConstructibleEventTarget_h_
+#define mozilla_dom_ConstructibleEventTarget_h_
+
+#include "mozilla/DOMEventTargetHelper.h"
+#include "js/RootingAPI.h"
+
+namespace mozilla {
+namespace dom {
+
+class ConstructibleEventTarget : public DOMEventTargetHelper
+{
+public:
+  // We're not worrying about ISupports and Cycle Collection here just for a wrapper function.
+  // This does mean that ConstructibleEventTarget will show up in CC and refcount logs as a
+  // DOMEventTargetHelper, but that's OK.
+
+  explicit ConstructibleEventTarget(nsIGlobalObject* aGlobalObject)
+    : DOMEventTargetHelper(aGlobalObject)
+  {
+  }
+
+  virtual JSObject* WrapObject(JSContext* cx,
+			       JS::Handle<JSObject*> aGivenProto) override;
+};
+
+} // namespace dom
+} // namespace mozilla
+
+#endif // mozilla_dom_ConstructibleEventTarget_h_