LibWeb+LibWebView+WebContent: Begin implementing simple site islotation

Site isolation is a common technique to reduce the chance that malicious
sites can access data from other sites. When the user navigates, we now
check if the target site is the same as the current site. If not, we
instruct the UI to perform the navigation in a new WebContent process.

The phrase "site" here is defined as the public suffix of the URL plus
one level. This means that navigating from "www.example.com" to
"sub.example.com" remains in the same process.

There's plenty of room for optimization around this. For example, we can
create a spare WebContent process ahead of time to hot-swap the target
site. We can also create a policy to keep the navigated-from process
around, in case the user quickly navigates back.

Libraries/LibWeb/HTML/Navigable.cpp

@@ -1349,6 +1349,12 @@ WebIDL::ExceptionOr<void> Navigable::navigate(NavigateParams params)
     auto& active_document = *this->active_document();
     auto& realm = active_document.realm();
 
+    // AD-HOC: If we are not able to continue in this process, request a new process from the UI.
+    if (!active_document.page().client().is_url_suitable_for_same_process_navigation(active_document.url(), params.url)) {
+        active_document.page().client().request_new_process_for_navigation(params.url);
+        return {};
+    }
+
     // 2. Let sourceSnapshotParams be the result of snapshotting source snapshot params given sourceDocument.
     auto source_snapshot_params = source_document->snapshot_source_snapshot_params();