Relax the isolation conflict log.

webrender/src/frame_builder.rs

@@ -323,7 +323,7 @@ impl FrameBuilder {
                 // must be drawn with a transparent background, unless the parent stacking context
                 // is the root of the page
                 let isolation = &mut self.stacking_context_store[parent_index.0].isolation;
-                if *isolation != ContextIsolation::None {
+                if *isolation == ContextIsolation::Items {
                     error!(
                         "Isolation conflict detected on {:?}: {:?}",
                         parent_index,