Backed out changeset 906c06cac59d (bug 1568126) for devtools tests timing out...

Backed out changeset 906c06cac59d (bug 1568126) for devtools tests timing out e.g browser_rules_flexbox-highlighter-on-mutation.js on a CLOSED TREE.

devtools/client/inspector/shared/highlighters-overlay.js

@@ -1104,7 +1104,7 @@ class HighlightersOverlay {
     }
 
     try {
-      const isInTree = await node.walkerFront.isInDOMTree(node);
+      const isInTree = await this.walker.isInDOMTree(node);
       if (!isInTree) {
         hideHighlighter(node);
       }