LibWeb: Fix document leak in SVGDecodedImageData

The SVGDecodedImageData creates a new Page and replaces its document
with a new one that contains SVG content. This change adds a destroy
call on the replaced document. Without this addition, all tasks
scheduled on the event loop during navigation, initiated while the
page's traversable is being created, will never execute, as the
initial replaced document will become inactive. This leads to a
document leak because the tasks use JS::Handle to hold document
pointer. Making the destroy call resolves the issue because it removes
all tasks associated with the destroyed document from the queue.

Userland/Libraries/LibWeb/SVG/SVGDecodedImageData.cpp

@@ -67,8 +67,10 @@ ErrorOr<NonnullRefPtr<SVGDecodedImageData>> SVGDecodedImageData::create(Page& ho
         .cross_origin_opener_policy = HTML::CrossOriginOpenerPolicy {},
         .about_base_url = {},
     };
+    // FIXME: Use Navigable::navigate() instead of manually replacing the navigable's document.
     auto document = DOM::Document::create_and_initialize(DOM::Document::Type::HTML, "text/html", navigation_params).release_value_but_fixme_should_propagate_errors();
     navigable->set_ongoing_navigation({});
+    navigable->active_document()->destroy();
     navigable->active_session_history_entry()->document_state->set_document(document);
 
     auto parser = HTML::HTMLParser::create_with_uncertain_encoding(document, data);