Bug 1586800 - Use the contextual WalkerFront in _deleteNode(). r=pbro

Differential Revision: https://phabricator.services.mozilla.com/D48439

devtools/client/inspector/markup/markup-context-menu.js

@@ -167,13 +167,15 @@ class MarkupContextMenu {
       return;
     }
 
+    const nodeFront = this.selection.nodeFront;
+
     // If the markup panel is active, use the markup panel to delete
     // the node, making this an undoable action.
     if (this.markup) {
-      this.markup.deleteNode(this.selection.nodeFront);
+      this.markup.deleteNode(nodeFront);
     } else {
       // remove the node from content
-      this.walker.removeNode(this.selection.nodeFront);
+      nodeFront.walkerFront.removeNode(nodeFront);
     }
   }