Bug 1497599 - Only run re-initialize moz-input-box during attributeChangedCallback if the attribute actually changed;r=paolo

This is unrelated to the other changesets in the bug, just a cleanup based on
patterns that have emerged in other attributeChangedCallbacks like in progressmeter

Differential Revision: https://phabricator.services.mozilla.com/D8174

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

toolkit/content/widgets/textbox.js

@@ -61,18 +61,17 @@ class MozInputBox extends MozXULElement {
   }
 
   attributeChangedCallback(name, oldValue, newValue) {
-    if (name === "spellcheck") {
-      this.spellcheck = newValue;
+    if (name === "spellcheck" && oldValue != newValue) {
       this._initUI();
     }
   }
 
   connectedCallback() {
-    this.spellcheck = this.hasAttribute("spellcheck");
     this._initUI();
   }
 
   _initUI() {
+    this.spellcheck = this.hasAttribute("spellcheck");
     if (this.menupopup) {
       this.menupopup.remove();
     }