Ladybird+WebContent: Update IPC calls to handle multiple traversables

The IPC layer between chromes and LibWeb now understands that multiple
top level traversables can live in each WebContent process.

This largely mechanical change adds a billion page_id/page_index
arguments to make sure that pages that end up opening new WebViews
through mechanisms like window.open() still work properly with those
extra windows.

Userland/Libraries/LibWebView/ViewImplementation.h

@@ -25,7 +25,7 @@ namespace WebView {
 
 class ViewImplementation {
 public:
-    virtual ~ViewImplementation() { }
+    virtual ~ViewImplementation();
 
     struct DOMNodeProperties {
         String computed_style_json;
@@ -192,6 +192,7 @@ protected:
 
     WebContentClient& client();
     WebContentClient const& client() const;
+    u64 page_id() const;
     virtual void update_zoom() = 0;
 
     enum class WindowResizeInProgress {