LibWeb: Put debug spam about negative content sizes behind a flag

We already clamp these values to zero, so it's actually pretty harmless
when this happens. If someone wants to investigate these issues deeper
and see if they can be fixed earlier in the layout pipeline, they can
enable the spam locally.

Userland/Libraries/LibWeb/Layout/LayoutState.cpp

@@ -1,9 +1,10 @@
 /*
- * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2022-2023, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Debug.h>
 #include <LibWeb/Layout/AvailableSpace.h>
 #include <LibWeb/Layout/BlockContainer.h>
 #include <LibWeb/Layout/LayoutState.h>
@@ -242,7 +243,7 @@ void LayoutState::UsedValues::set_content_width(CSSPixels width)
 {
     if (width < 0) {
         // Negative widths are not allowed in CSS. We have a bug somewhere! Clamp to 0 to avoid doing too much damage.
-        dbgln("FIXME: Layout calculated a negative width for {}: {}", m_node->debug_description(), width);
+        dbgln_if(LIBWEB_CSS_DEBUG, "FIXME: Layout calculated a negative width for {}: {}", m_node->debug_description(), width);
         width = 0;
     }
     m_content_width = width;
@@ -253,7 +254,7 @@ void LayoutState::UsedValues::set_content_height(CSSPixels height)
 {
     if (height < 0) {
         // Negative heights are not allowed in CSS. We have a bug somewhere! Clamp to 0 to avoid doing too much damage.
-        dbgln("FIXME: Layout calculated a negative height for {}: {}", m_node->debug_description(), height);
+        dbgln_if(LIBWEB_CSS_DEBUG, "FIXME: Layout calculated a negative height for {}: {}", m_node->debug_description(), height);
         height = 0;
     }
     m_content_height = height;