[Switch Access] Add waitForPredicate to test base

Add a function to the Switch Access end-to-end test base to wait until
a tree change happens, after which a specified predicate function
return true.

This function is not yet used, but will be in a follow-up change.

AX-Relnotes: n/a.
Bug: None
Change-Id: I12802fc50cae80463f1b28adfd573abd0dacfa5b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2203641
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770349}

chrome/browser/resources/chromeos/accessibility/switch_access/switch_access_e2e_test_base.js

@@ -113,6 +113,27 @@ SwitchAccessE2ETest.prototype = {
     return this.findNodeMatchingPredicate(predicate, nodeString);
   },
 
+  /**
+   * @param {function(): boolean} predicate The condition under which the
+   *     callback should be fired.
+   * @param {function()} callback
+   */
+  waitForPredicate(predicate, callback) {
+    if (predicate()) {
+      callback();
+      return;
+    }
+    const listener = () => {
+      if (predicate()) {
+        NavigationManager.desktopNode.removeEventListener(
+            'childrenChanged', listener, false /* capture */);
+        callback();
+      }
+    };
+    NavigationManager.desktopNode.addEventListener(
+        'childrenChanged', listener, false /* capture */);
+  },
+
   /**
    * From chromevox_next_e2e_test_base.js
    * Gets the desktop from the automation API and Launches a new tab with