LibWeb: Remove uses of obsolete PlatformObject::global_object()

This API is a relic from the time when it was important for objects to
have easy access to the Window, and to know it was the global object.

We now have more spec-related concepts like relevant_global_object and
current_global_object to pull the Window out of thin air.

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.cpp

@@ -175,7 +175,7 @@ Variant<JS::Handle<DOM::Element>, JS::Handle<DOM::Document>> IntersectionObserve
     }
 
     // otherwise, it is the top-level browsing context’s document node, referred to as the implicit root.
-    return JS::make_handle(global_object().page().top_level_browsing_context().active_document());
+    return JS::make_handle(verify_cast<HTML::Window>(HTML::relevant_global_object(*this)).page().top_level_browsing_context().active_document());
 }
 
 // https://www.w3.org/TR/intersection-observer/#intersectionobserver-root-intersection-rectangle