LibWeb: Update layout in dump_dom_tree_as_json()

Ensure layout is up to date before serializing the DOM tree as JSON for
the inspector, so that layout information is accurate.

Libraries/LibWeb/DOM/Document.cpp

@@ -3526,6 +3526,8 @@ void Document::set_bg_color(String const& value)
 
 String Document::dump_dom_tree_as_json() const
 {
+    const_cast<Document&>(*this).update_layout(UpdateLayoutReason::InspectDOMTree);
+
     StringBuilder builder;
     auto json = MUST(JsonObjectSerializer<>::try_create(builder));
     serialize_tree_as_json(json);

Libraries/LibWeb/DOM/Document.h

@@ -107,6 +107,7 @@ enum class InvalidateLayoutTreeReason {
     X(HTMLImageElementY)                   \
     X(HTMLInputElementHeight)              \
     X(HTMLInputElementWidth)               \
+    X(InspectDOMTree)                      \
     X(InternalsHitTest)                    \
     X(MediaQueryListMatches)               \
     X(NavigableSelectedText)               \