a11y/Mac: Add screenreader support for SubtleNotificationView announcements.

chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm

Index: chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
diff --git a/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm b/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
index 8b10f31e9b108ff3867bb97891aaa0b86e793f35..5d16eefeb89348764243e42d59ef6eeaf255148c 100644
--- a/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
+++ b/chrome/browser/ui/cocoa/omnibox/omnibox_view_mac.mm
@@ -7,6 +7,7 @@
 #include <Carbon/Carbon.h>  // kVK_Return
 
 #include "base/mac/foundation_util.h"
+#import "base/mac/sdk_forward_declarations.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram.h"
 #include "base/strings/string_util.h"
@@ -47,9 +48,6 @@
 #include "ui/gfx/font_list.h"
 #include "ui/gfx/geometry/rect.h"
 
-// TODO(ellyjones): Remove this when the deployment target is 10.9 or later.
-extern NSString* const NSAccessibilityPriorityKey;
-
 using content::WebContents;
 
 // Focus-handling between |field_| and model() is a bit subtle.