Bug 1496478 - Only change the search engine when scrolling on searchbox if accel key is pressed;r=adw

The Custom Element migration missed the [modifiers=accel] in XBL on the DOMMouseScroll handler.

Differential Revision: https://phabricator.services.mozilla.com/D7780

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

browser/components/search/content/searchbar.js

@@ -404,7 +404,11 @@ class MozSearchbar extends MozXULElement {
       this.select();
     });
 
-    this.addEventListener("DOMMouseScroll", (event) => { this.selectEngine(event, (event.detail > 0)); }, true);
+    this.addEventListener("DOMMouseScroll", (event) => {
+      if (event.getModifierState("Accel")) {
+        this.selectEngine(event, event.detail > 0);
+      }
+    }, true);
 
     this.addEventListener("input", (event) => { this.updateGoButtonVisibility(); });