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 346cb7168571392f59d609106ff894c08bd0b7d8..5e278030e1743a5a435b70fa8983af26be0db792 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"
 #include "ui/accessibility/ax_node_data.h"
 #include "ui/accessibility/ax_view_state.h"
@@ -508,6 +509,18 @@ void AXPlatformNodeMac::NotifyAccessibilityEvent(ui::AXEvent event_type) {
   // Add mappings between ui::AXEvent and NSAccessibility notifications using
   // the EventMap above. This switch contains exceptions to those mappings.
   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);
+      return;
+    }
     case ui::AX_EVENT_TEXT_CHANGED:
       // If the view is a user-editable textfield, this should change the value.
       if (GetData().role == ui::AX_ROLE_TEXT_FIELD) {