[Switch Access] Fix bug where menu had sub-groupings rendering it unusable.

In crrev.com/c/2303458 a bug was introduced which added sub-groupings
within the Switch Access menu. This seems to be related to the change
to use an EventHandler in switch_access.js which always acts on event.target,
whereas previously it sometimes did an action on one of the children
of event.target.

Although I don't fully understand the logic, reverting this one file
from that change does restore the correct behavior.

AX-Relnotes: N/A
Bug: 1109355
Change-Id: Id7feb1a2474e06a64d0b371beade6e33800d382c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318315
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Anastasia Helfinstein <anastasi@google.com>
Cr-Commit-Position: refs/heads/master@{#792027}

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

@@ -65,26 +65,35 @@ class SwitchAccess {
     }
     // If it's not currently in the tree, listen for changes to the desktop
     // tree.
-    const eventPredicate = (event) => {
+    const onDesktopChildrenChanged = (event) => {
       if (predicate(event.target)) {
-        return true;
+        // If the event target is the node we're looking for, we've found it.
+        desktop.removeEventListener(
+            chrome.automation.EventType.CHILDREN_CHANGED,
+            onDesktopChildrenChanged, false);
+        foundCallback(event.target);
       } else if (event.target.children.length > 0) {
-        // See if one of its children is the node we're looking for.
+        // Otherwise, see if one of its children is the node we're looking for.
         const treeWalker = new AutomationTreeWalker(
             event.target, constants.Dir.FORWARD,
             {visit: predicate, root: (node) => node == event.target});
         treeWalker.next();
-        return !!treeWalker.node;
-      } else {
-        return false;
+        if (treeWalker.node) {
+          desktop.removeEventListener(
+              chrome.automation.EventType.CHILDREN_CHANGED,
+              onDesktopChildrenChanged, false);
+          foundCallback(treeWalker.node);
+        }
       }
     };
 
-    new EventHandler(
-        desktop, chrome.automation.EventType.CHILDREN_CHANGED,
-        (event) => foundCallback(event.target),
-        {listenOnce: true, predicate: eventPredicate})
-        .start();
+    // Note: Cannot use an EventHandler here because in some cases we want
+    // to run foundCallback on the event.target, and in some cases we want to
+    // run foundCallback on one of the target's children. We would need to
+    // recalculate the interesting child twice to use EventHandler.
+    desktop.addEventListener(
+        chrome.automation.EventType.CHILDREN_CHANGED, onDesktopChildrenChanged,
+        false);
   }
 
   /*