Consolidate SDK forwards in sdk_forward_declarations.h.

ui/message_center/cocoa/popup_controller.mm

Index: ui/message_center/cocoa/popup_controller.mm
diff --git a/ui/message_center/cocoa/popup_controller.mm b/ui/message_center/cocoa/popup_controller.mm
index 30badc79bfe10baa78139c4263e9d5c43aa73d98..0f22937b86db7790a7c9e86c6e85332157d948d6 100644
--- a/ui/message_center/cocoa/popup_controller.mm
+++ b/ui/message_center/cocoa/popup_controller.mm
@@ -13,13 +13,6 @@
 #import "ui/message_center/cocoa/popup_collection.h"
 #include "ui/message_center/message_center.h"
 
-#if !defined(MAC_OS_X_VERSION_10_7) || \
-    MAC_OS_X_VERSION_MAX_ALLOWED < MAC_OS_X_VERSION_10_7
-enum {
-  NSWindowCollectionBehaviorFullScreenAuxiliary = 1 << 8
-};
-#endif  // MAC_OS_X_VERSION_10_7
-
 ////////////////////////////////////////////////////////////////////////////////
 
 @interface MCPopupController (Private)