Fix flakiness in accessibility hit testing browser tests

The cause of the flakiness is that we weren't waiting long enough for
the pinch zoom change to be reflected in the accessibility tree.
Waiting for kLocationChanged appears to address the issue - with this
change I was able to run the flaky test case 1000x without a single
failure.

Bug: 1069850, 1070647
Change-Id: If0cba1804776125885db3d0a2aedfeeca9340048
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2155077Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Kevin Babbitt <kbabbitt@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#760577}

content/browser/accessibility/hit_testing_browsertest.cc

@@ -224,7 +224,8 @@ class AccessibilityHitTestingBrowserTest
   void SimulatePinchZoom(float desired_page_scale) {
     RenderFrameSubmissionObserver observer(shell()->web_contents());
     AccessibilityNotificationWaiter accessibility_waiter(
-        shell()->web_contents(), ui::AXMode(), ax::mojom::Event::kNone);
+        shell()->web_contents(), ui::AXMode(),
+        ax::mojom::Event::kLocationChanged);
 
     const gfx::Rect contents_rect =
         shell()->web_contents()->GetContainerBounds();