LibWeb: Add FIXME about definiteness to UsedValues::set_content_width()

Fixing this function will be quite an undertaking since a *lot* of code
relies on set_content_width() implicitly flipping the definiteness of
the width. It is wrong though, so we do need to fix it eventually.

Userland/Libraries/LibWeb/Layout/LayoutState.cpp

@@ -495,6 +495,8 @@ void LayoutState::UsedValues::set_content_width(CSSPixels width)
         width = 0;
     }
     m_content_width = width;
+    // FIXME: We should not do this! Definiteness of widths should be determined early,
+    //        and not changed later (except for some special cases in flex layout..)
     m_has_definite_width = true;
 }