Bug 1536819 - Fix node.offsetParent is null error in Rule View r=gl

The editing of selectors in the rule view is broken (see https://bugzil.la/1536867) but that is not related to this issue.

Differential Revision: https://phabricator.services.mozilla.com/D24210

--HG--
extra : moz-landing-system : lando

devtools/client/inspector/rules/rules.js

@@ -450,15 +450,10 @@ CssRuleView.prototype = {
   },
 
   /**
-   * Retrieve the RuleEditor instance that should be stored on
-   * the offset parent of the node
+   * Retrieve the RuleEditor instance.
    */
   _getRuleEditorForNode: function(node) {
-    if (!node.offsetParent) {
-      // some nodes don't have an offsetParent, but their parentNode does
-      node = node.parentNode;
-    }
-    return node.offsetParent._ruleEditor;
+    return node.closest(".ruleview-rule")._ruleEditor;
   },
 
   /**