Fix DumpAccessibilityEvents tests on Mac and improve test coverage

content/browser/accessibility/dump_accessibility_browsertest_base.cc

Index: content/browser/accessibility/dump_accessibility_browsertest_base.cc
diff --git a/content/browser/accessibility/dump_accessibility_browsertest_base.cc b/content/browser/accessibility/dump_accessibility_browsertest_base.cc
index 5e0f88bda2ff94cafda1cc4a287b140148ca14df..2a02cd515e9394444328c74856add4efd8cdeeab 100644
--- a/content/browser/accessibility/dump_accessibility_browsertest_base.cc
+++ b/content/browser/accessibility/dump_accessibility_browsertest_base.cc
@@ -195,10 +195,11 @@ void DumpAccessibilityTestBase::RunTestForPlatform(
   BrowserAccessibilityStateImpl::GetInstance()->
       set_disable_hot_tracking_for_testing(true);
 
-  // Normally we avoid firing accessibility focus events when the containing
-  // native window isn't focused. Suppress this check to avoid flaky tests,
-  // since we can't always control when a native window loses focus.
-  BrowserAccessibilityManager::NeverSuppressFocusEventsForTesting();
+  // Normally some accessibility events that would be fired are suppressed or
+  // delayed, depending on what has focus or the type of event. For testing,
+  // we want all events to fire immediately to make tests predictable and not
+  // flaky.
+  BrowserAccessibilityManager::NeverSuppressOrDelayEventsForTesting();
 
   NavigateToURL(shell(), GURL(url::kAboutBlankURL));