Handle uncommon download verdict for safebrowsing download protection.

chrome/common/safe_browsing/csd.proto

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Client side phishing and malware detection request and response
 // protocol buffers.  Those protocol messages should be kept in sync
 // with the server implementation.
 //
 // If you want to change this protocol definition or you have questions
 // regarding its format please contact chrome-anti-phishing@googlegroups.com.
@@ skipping 148 matching lines @@
   optional bool user_initiated = 6;
 }
 
 message ClientDownloadResponse {
   enum Verdict {
     // Download is considered safe.
     SAFE = 0;
     // Download is considered dangerous.  Chrome should show a warning to the
     // user.
     DANGEROUS = 1;
+    // Download is unknown.  Chrome should display a less severe warning.
+    UNCOMMON = 2;
   }
   required Verdict verdict = 1;
 }