[a11y switch access] Avoid using obsolete child node as the focus node

When fetching history record, the group stored in history updates. As a
result, the focus node stored in history may not be a child of the group
anymore. However, that obsolete node is still used as the preferred
focus node. This CL fixes it. See comments under the issue for detailed
analysis.

Bug: 831407
Change-Id: I1e1553c4322408fb431b212d9e0397b95e7f50ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2482184Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#818928}

chrome/browser/resources/chromeos/accessibility/switch_access/navigation_manager.js

@@ -434,9 +434,23 @@ class NavigationManager {
    */
   restoreFromHistory_() {
     const data = this.history_.retrieve();
+
+    // |data.focus| may not be a child of |data.group| anymore since
+    // |data.group| updates when retrieving the history record. So |data.focus|
+    // should not be used as the preferred focus node.
+    const groupChildren = data.group.children;
+    var focusTarget = null;
+    for (var index = 0; index < groupChildren.length; ++index) {
+      const child = groupChildren[index];
+      if (child.isEquivalentTo(data.focus)) {
+        focusTarget = child;
+        break;
+      }
+    }
+
     // retrieve() guarantees that the group is valid, but not the focus.
-    if (data.focus.isValidAndVisible()) {
-      this.setGroup_(data.group, data.focus);
+    if (focusTarget && focusTarget.isValidAndVisible()) {
+      this.setGroup_(data.group, focusTarget);
     } else {
       this.setGroup_(data.group);
     }