Speculative fix for MagnifierE2ETest.MovesDockedMagnifierToActiveDescendant

The test was failing/flaky on some MSAN bots, presumably because the
listener was registered after the event was fired.

Introduce a helper class that allows us to easily register the listener,
perform the event, then wait for the listener to have fired.

TBR=josiahk@chromium.org, dtseng@chromium.org

Bug: 1145612
Test: Runs locally but real test will be whether this fixes failures on
the 'Linux ChromiumOS MSan Tests bot'

Change-Id: I16303d96b101113e80ab4f484b4e604faeab4c88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2560622Reviewed-by: Pavol Marko <pmarko@chromium.org>
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830991}

chrome/browser/resources/chromeos/accessibility/accessibility_common/magnifier/docked_magnifier_test.js

@@ -14,18 +14,33 @@ DockedMagnifierE2ETest = class extends E2ETestBase {
   constructor() {
     super();
     window.RoleType = chrome.automation.RoleType;
-  }
 
-  async getNextMagnifierBounds() {
-    return new Promise(resolve => {
-      const listener = (magnifierBounds) => {
-        chrome.accessibilityPrivate.onMagnifierBoundsChanged.removeListener(
-            listener);
-        resolve(magnifierBounds);
-      };
-      chrome.accessibilityPrivate.onMagnifierBoundsChanged.addListener(
-          listener);
-    });
+    /**
+     * Registers a listener for
+     * chrome.accessibilityPrivate.onMagnifierBoundsChanged in the ctor. After
+     * that, the test code can perform the action, and then use
+     * waitForNextMagnifierBounds() to be wait for the previously-registered
+     * listener to be called.
+     *
+     * @private
+     */
+    this.nextMagnifierBoundsWaiter_ = class {
+      constructor() {
+        this.promise_ = new Promise(resolve => {
+          const listener = (magnifierBounds) => {
+            chrome.accessibilityPrivate.onMagnifierBoundsChanged.removeListener(
+                listener);
+            resolve(magnifierBounds);
+          };
+          chrome.accessibilityPrivate.onMagnifierBoundsChanged.addListener(
+              listener);
+        });
+      }
+
+      async waitForNextMagnifierBounds() {
+        return this.promise_;
+      }
+    };
   }
 
   /** @override */
@@ -60,18 +75,18 @@ TEST_F(
         const group = root.find({role: RoleType.GROUP});
 
         // Focus and move magnifier to top.
+        // Then verify magnifier contain top and not banana.
+        const boundsWaiter1 = new this.nextMagnifierBoundsWaiter_();
         top.focus();
-
-        // Verify magnifier contain top and not banana.
-        let bounds = await this.getNextMagnifierBounds();
+        let bounds = await boundsWaiter1.waitForNextMagnifierBounds();
         assertTrue(RectUtil.contains(bounds, top.location));
         assertFalse(RectUtil.contains(bounds, banana.location));
 
         // Click group to change active descendant to banana.
+        // Then verify magnifier bounds contain banana.
+        const boundsWaiter2 = new this.nextMagnifierBoundsWaiter_();
         group.doDefault();
-
-        // Verify magnifier bounds contain banana.
-        bounds = await this.getNextMagnifierBounds();
+        bounds = await boundsWaiter2.waitForNextMagnifierBounds();
         assertFalse(RectUtil.contains(bounds, top.location));
         assertTrue(RectUtil.contains(bounds, banana.location));
       });