a11y/Mac: Add screenreader support for SubtleNotificationView announcements.

ui/accessibility/platform/ax_platform_node_mac.mm

Index: ui/accessibility/platform/ax_platform_node_mac.mm
diff --git a/ui/accessibility/platform/ax_platform_node_mac.mm b/ui/accessibility/platform/ax_platform_node_mac.mm
index 620841889eae237edc5ee3a5a3f4f237f6de4a33..ac76cb71af5e5421f05f857c109219725602c676 100644
--- a/ui/accessibility/platform/ax_platform_node_mac.mm
+++ b/ui/accessibility/platform/ax_platform_node_mac.mm
@@ -8,6 +8,7 @@
 #include <stddef.h>
 
 #include "base/macros.h"
+#import "base/mac/sdk_forward_declarations.h"
 #include "base/strings/sys_string_conversions.h"
 #import "ui/accessibility/ax_node_data.h"
 #import "ui/accessibility/platform/ax_platform_node_delegate.h"
@@ -335,7 +336,22 @@ gfx::NativeViewAccessible AXPlatformNodeMac::GetNativeViewAccessible() {
 }
 
 void AXPlatformNodeMac::NotifyAccessibilityEvent(ui::AXEvent event_type) {
-  // TODO(dmazzoni): implement this.  http://crbug.com/396137
+  switch (event_type) {
+    case ui::AX_EVENT_ALERT: {
+      NSString* announcement = base::SysUTF8ToNSString(
+          GetData().GetStringAttribute(ui::AX_ATTR_NAME));
+      NSDictionary* notification_info = @{
+        NSAccessibilityAnnouncementKey : announcement,
+        NSAccessibilityPriorityKey : @(NSAccessibilityPriorityHigh)
+      };
+      NSAccessibilityPostNotificationWithUserInfo(
+          [NSApp mainWindow], NSAccessibilityAnnouncementRequestedNotification,
+          notification_info);
+    } break;
+    default:
+      // TODO(dmazzoni): implement this.  http://crbug.com/396137
+      break;
+  }
 }
 
 int AXPlatformNodeMac::GetIndexInParent() {