Revert "Speculative fix for MagnifierE2ETest.MovesDockedMagnifierToActiveDescendant"

This reverts commit a92eb9b7.

Reason for revert: Looks like the test failure is still there for Linux ChromiumOS MSan Tests and Linux Chromium OS ASan LSan Tests (1)
First fail occurance: https://ci.chromium.org/p/chromium/builders/ci/Linux%20Chromium%20OS%20ASan%20LSan%20Tests%20%281%29/38932 and https://ci.chromium.org/p/chromium/builders/ci/Linux%20ChromiumOS%20MSan%20Tests/21447
Revert this CL so I can revert https://crrev.com/c/2552163

Original change's description:
> 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/+/2560622
> Reviewed-by: Pavol Marko <pmarko@chromium.org>
> Commit-Queue: Pavol Marko <pmarko@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#830991}

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

Change-Id: I609372a222b15106cd1bf3c2c7bb10758214e136
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1145612
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2560446Reviewed-by: Maggie Cai <mxcai@chromium.org>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831244}

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

@@ -14,19 +14,10 @@ DockedMagnifierE2ETest = class extends E2ETestBase {
   constructor() {
     super();
     window.RoleType = chrome.automation.RoleType;
+  }
 
-    /**
-     * 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 => {
+  async getNextMagnifierBounds() {
+    return new Promise(resolve => {
       const listener = (magnifierBounds) => {
         chrome.accessibilityPrivate.onMagnifierBoundsChanged.removeListener(
             listener);
@@ -37,12 +28,6 @@ DockedMagnifierE2ETest = class extends E2ETestBase {
     });
   }
 
-      async waitForNextMagnifierBounds() {
-        return this.promise_;
-      }
-    };
-  }
-
   /** @override */
   testGenCppIncludes() {
     super.testGenCppIncludes();
@@ -75,18 +60,18 @@ 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();
-        let bounds = await boundsWaiter1.waitForNextMagnifierBounds();
+
+        // Verify magnifier contain top and not banana.
+        let bounds = await this.getNextMagnifierBounds();
         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();
-        bounds = await boundsWaiter2.waitForNextMagnifierBounds();
+
+        // Verify magnifier bounds contain banana.
+        bounds = await this.getNextMagnifierBounds();
         assertFalse(RectUtil.contains(bounds, top.location));
         assertTrue(RectUtil.contains(bounds, banana.location));
       });