LibWeb: Expose window.self and window.frames

"self" is a way to refer to the global object that will work in both
a window context and a web worker context.

"frames" apparently used to return a list of frame objects according
to MDN, but it now just returns the window object.

Libraries/LibWeb/Tests/Window/window.window_frames_self.js

@@ -0,0 +1,8 @@
+loadPage("file:///res/html/misc/blank.html");
+
+afterInitialPageLoad(() => {
+    test("window.{window,frames,self} all return the Window object", () => {
+        expect(window.window).toBe(window.frames);
+        expect(window.window).toBe(window.self);
+    });
+});