[Switch Access] Copy array more efficiently

Instead of iterating through the array to make a copy, use the ES6
spread operator.

AX-Relnotes: n/a.
Bug: None.
Change-Id: Ia0e1a8d738c5c9480bf991ba9de4a50bb4fca482
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2391506
Auto-Submit: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Akihiro Ota <akihiroota@chromium.org>
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#807242}

chrome/browser/resources/chromeos/accessibility/switch_access/nodes/group_node.js

@@ -51,10 +51,8 @@ class GroupNode extends SAChildNode {
   asRootNode() {
     const root = new SARootNode();
 
-    const children = [];
-    for (const child of this.children_) {
-      children.push(child);
-    }
+    // Make a copy of the children array.
+    const children = [...this.children_];
 
     children.push(new BackButtonNode(root));
     root.children = children;