Hide RenderBox::isBox() / RenderText::isText()

Source/core/rendering/RenderBox.h

Index: Source/core/rendering/RenderBox.h
diff --git a/Source/core/rendering/RenderBox.h b/Source/core/rendering/RenderBox.h
index 235b00fcc1899b88e68e7e233da9242e33a5ace0..b8465a1786f51f49d83b27e4420a1b0545d40462 100644
--- a/Source/core/rendering/RenderBox.h
+++ b/Source/core/rendering/RenderBox.h
@@ -731,7 +731,8 @@ private:
 
     void invalidatePaintIfNeeded(const PaintInvalidationState&, const RenderLayerModelObject& newPaintInvalidationContainer);
 
-private:
+    bool isBox() const WTF_DELETED_FUNCTION; // This will catch anyone doing an unnecessary check.
+
     // The width/height of the contents + borders + padding.  The x/y location is relative to our container (which is not always our parent).
     LayoutRect m_frameRect;