LibWeb: Add `m_is_grid_item` in `Layout::Node`

This information is going to be needed during painting to treat grid
items as stacking contexts.

Userland/Libraries/LibWeb/Layout/GridFormattingContext.cpp

@@ -1328,6 +1328,8 @@ void GridFormattingContext::place_grid_items(AvailableSpace const& available_spa
         if (child_box.is_out_of_flow(*this))
             return IterationDecision::Continue;
 
+        child_box.set_grid_item(true);
+
         auto& order_bucket = order_item_bucket.ensure(child_box.computed_values().order());
         order_bucket.append(child_box);
 

Userland/Libraries/LibWeb/Layout/Node.h

@@ -124,6 +124,9 @@ public:
     bool is_flex_item() const { return m_is_flex_item; }
     void set_flex_item(bool b) { m_is_flex_item = b; }
 
+    bool is_grid_item() const { return m_is_grid_item; }
+    void set_grid_item(bool b) { m_is_grid_item = b; }
+
     Box const* containing_block() const;
     Box* containing_block() { return const_cast<Box*>(const_cast<Node const*>(this)->containing_block()); }
 
@@ -194,6 +197,8 @@ private:
     SelectionState m_selection_state { SelectionState::None };
 
     bool m_is_flex_item { false };
+    bool m_is_grid_item { false };
+
     GeneratedFor m_generated_for { GeneratedFor::NotGenerated };
 };