Added a11y alert on entering overview mode

ash/accessibility_delegate.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ASH_ACCESSIBILITY_DELEGATE_H_
 #define ASH_ACCESSIBILITY_DELEGATE_H_
 
 #include "ash/ash_export.h"
 #include "ash/magnifier/magnifier_constants.h"
 #include "base/time/time.h"
 
 namespace ash {
 
 enum AccessibilityNotificationVisibility {
   A11Y_NOTIFICATION_NONE,
   A11Y_NOTIFICATION_SHOW,
 };
 
 enum AccessibilityAlert {
   A11Y_ALERT_NONE,
-  A11Y_ALERT_WINDOW_NEEDED
+  A11Y_ALERT_WINDOW_NEEDED,
+  A11Y_ALERT_WINDOW_OVERVIEW_MODE_ENTERED
 };
 
 // A delegate class to control and query accessibility features.
 class ASH_EXPORT AccessibilityDelegate {
  public:
   virtual ~AccessibilityDelegate() {}
 
   // Invoked to toggle spoken feedback for accessibility
   virtual void ToggleSpokenFeedback(
       AccessibilityNotificationVisibility notify) = 0;
@@ skipping 59 matching lines @@
   // Gets the last accessibility alert that was triggered.
   virtual AccessibilityAlert GetLastAccessibilityAlert() = 0;
 
   // Initiates play of shutdown sound and returns it's duration.
   virtual base::TimeDelta PlayShutdownSound() const = 0;
 };
 
 }  // namespace ash
 
 #endif  // ASH_ACCESSIBILITYDELEGATE_H_