LibWeb: Correctly initialize Storage objects on the Document

Instead of storing all storage objects in static memory, we now
follow the the spec by lazily creating a unique Storage object
on each document object.

Each Storage object now holds a 'proxy' to the underlying backing
storage. For now, this proxy is simply a reference to the backing
object. In the future, it will need to be some type of interface
object that stores on a SQLite database or similar.

Session storage is now correctly stored / tracked as part of the
TraversableNavigable object.

Local storage is still stored in a static map, but eventually this
should be factored into something that is stored at the user agent
level.

Libraries/LibWeb/HTML/TraversableNavigable.h

@@ -13,6 +13,7 @@
 #include <LibWeb/HTML/VisibilityState.h>
 #include <LibWeb/Page/Page.h>
 #include <LibWeb/Painting/DisplayListPlayerSkia.h>
+#include <LibWeb/StorageAPI/StorageShed.h>
 #include <WebContent/BackingStoreManager.h>
 
 #ifdef AK_OS_MACOS
@@ -107,6 +108,9 @@ public:
 
     RefPtr<Gfx::SkiaBackendContext> skia_backend_context() const { return m_skia_backend_context; }
 
+    StorageAPI::StorageShed& storage_shed() { return m_storage_shed; }
+    StorageAPI::StorageShed const& storage_shed() const { return m_storage_shed; }
+
 private:
     TraversableNavigable(GC::Ref<Page>);
 
@@ -142,6 +146,10 @@ private:
     // https://html.spec.whatwg.org/multipage/document-sequences.html#system-visibility-state
     VisibilityState m_system_visibility_state { VisibilityState::Hidden };
 
+    // https://storage.spec.whatwg.org/#traversable-navigable-storage-shed
+    // A traversable navigable holds a storage shed, which is a storage shed. A traversable navigable’s storage shed holds all session storage data.
+    StorageAPI::StorageShed m_storage_shed;
+
     GC::Ref<SessionHistoryTraversalQueue> m_session_history_traversal_queue;
 
     String m_window_handle;