style: Fix propagation of the can_be_fragmented bit on Servo.

This was bogus before, but seems like the right thing to do.

components/style/matching.rs

@@ -413,6 +413,15 @@ trait PrivateMatchMethods: TElement {
             }
         }
 
+        #[cfg(feature = "servo")]
+        {
+            // We may need to set or propagate the CAN_BE_FRAGMENTED bit
+            // on our children.
+            if old_values.is_multicol() != new_values.is_multicol() {
+                return ChildCascadeRequirement::MustCascadeChildren;
+            }
+        }
+
         // We could prove that, if our children don't inherit reset
         // properties, we can stop the cascade.
         ChildCascadeRequirement::MustCascadeChildrenIfInheritResetStyle