LibWeb: Forbid sharing image requests across documents

This change addresses the bug where images unable to load when the
reload button in the UI is clicked repeatedly. Before this fix, it was
possible to use SharedImageRequests across multiple documents. However,
when the document that initiated the request is gone, tasks scheduled
on the event loop remain in the fetching state because the originating
document is no longer active. Furthermore, another reason to prohibit
the sharing of image requests across documents is that the "Origin"
header in an image request is dependent on the document.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -3555,4 +3555,9 @@ void Document::update_for_history_step_application(JS::NonnullGCPtr<HTML::Sessio
     }
 }
 
+HashMap<AK::URL, HTML::SharedImageRequest*>& Document::shared_image_requests()
+{
+    return m_shared_image_requests;
+}
+
 }