LibWeb+WebContent: Plumb ability for WebContent process to hold N pages

In order for same-origin NavigableContainers (iframe, frame, embed, ...)
and window.open() WindowProxies to have the proper JS access to their
embedder/opener, we need to host multiple top level traversables in the
same WebContent process. As a first step, make WebContent::PageHost hold
a HashMap of PageClient objects, each holding their own Web::Page that
represents a TraversableNavigable's API surface with the UI process.

Userland/Libraries/LibWeb/Page/Page.h

@@ -46,6 +46,8 @@ public:
     PageClient& client() { return m_client; }
     PageClient const& client() const { return m_client; }
 
+    void set_top_level_traversable(JS::NonnullGCPtr<HTML::TraversableNavigable>);
+
     // FIXME: This is a hack.
     bool top_level_traversable_is_initialized() const;