Bug 1696717 - Mark GeckoMVMContext final. r=hiro

Again, just drive-by cleanup.

Depends on D107363

Differential Revision: https://phabricator.services.mozilla.com/D107364

layout/base/GeckoMVMContext.h

@@ -23,7 +23,7 @@ class EventTarget;
  * This is intended for production use (whereas TestMVMContext is intended for
  * testing.)
  */
-class GeckoMVMContext : public MVMContext {
+class GeckoMVMContext final : public MVMContext {
  public:
   explicit GeckoMVMContext(dom::Document* aDocument, PresShell* aPresShell);
   void AddEventListener(const nsAString& aType, nsIDOMEventListener* aListener,