Disallow repeated PushManager.subscribes with different sender ids

content/public/common/push_messaging_status.cc

Index: content/public/common/push_messaging_status.cc
diff --git a/content/public/common/push_messaging_status.cc b/content/public/common/push_messaging_status.cc
index 09a56f4be8645bb88ca4732f5964fb464166ad49..abee3207eff2ac45fd04b4a11b627257ebf74c73 100644
--- a/content/public/common/push_messaging_status.cc
+++ b/content/public/common/push_messaging_status.cc
@@ -52,6 +52,11 @@ const char* PushRegistrationStatusToString(PushRegistrationStatus status) {
     case PUSH_REGISTRATION_STATUS_MANIFEST_EMPTY_OR_MISSING:
       return "Registration failed - missing applicationServerKey, and manifest "
              "empty or missing";
+
+    case PUSH_REGISTRATION_STATUS_SENDER_ID_MISMATCH:
+      return "Registration failed - A subscription with a different "
+             "applicationServerKey (or gcm_sender_id) already exists; to "
+             "change the applicationServerKey, unsubscribe then resubscribe.";
   }
   NOTREACHED();
   return "";