Added a11y alert on entering overview mode

ash/wm/overview/window_overview.cc

Index: ash/wm/overview/window_overview.cc
diff --git a/ash/wm/overview/window_overview.cc b/ash/wm/overview/window_overview.cc
index ec5efc76a6bc8036a206850c48ed95be0ac1607e..063b3e9663d4db6f0537a7395a249140cfec53e5 100644
--- a/ash/wm/overview/window_overview.cc
+++ b/ash/wm/overview/window_overview.cc
@@ -6,6 +6,7 @@
 
 #include <algorithm>
 
+#include "ash/accessibility_delegate.h"
 #include "ash/metrics/user_metrics_recorder.h"
 #include "ash/screen_util.h"
 #include "ash/shell.h"
@@ -150,6 +151,8 @@ WindowOverview::WindowOverview(WindowSelector* window_selector,
   shell->GetScreen()->AddObserver(this);
   shell->metrics()->RecordUserMetricsAction(UMA_WINDOW_OVERVIEW);
   HideAndTrackNonOverviewWindows();
+  Shell::GetInstance()->accessibility_delegate()->TriggerAccessibilityAlert(

[flackr, 2014/04/28 23:34:42]

nit: Re-use local "shell" variable.

Also, while overview from alt tab is likely to go away, I'd prefer if we only
deliver this notification if window_selector_->mode() ==
WindowSelector::OVERVIEW since I don't think this notification makes sense when
we auto enter overview from alt-tab cycling.

[Nina, 2014/04/29 13:12:17]

Done.

+      A11Y_ALERT_WINDOW_OVERVIEW_MODE_ENTERED);
 }
 
 WindowOverview::~WindowOverview() {