DownloadProtection: Add more graceful handling of verdict=UNKNOWN

chrome/common/safe_browsing/csd.proto

 // Copyright (c) 2012 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.
 //
 // This proto file includes:
 // (1) Client side phishing and malware detection request and response
 //   protocol buffers.  Those protocol messages should be kept in sync
 //   with the server implementation.
 //
 // (2) Safe Browsing reporting protocol buffers.
@@ skipping 372 matching lines @@
   repeated string alternate_extensions = 32;
 }
 
 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.
+    // Download is uncommon.  Chrome should display a less severe warning.
     UNCOMMON = 2;
     // The download is potentially unwanted.
     POTENTIALLY_UNWANTED = 3;
     // The download is from a dangerous host.
     DANGEROUS_HOST = 4;
+    // The backend doesn't have confidence in its verdict of this file.
+    // Chrome should show the default warning if configured for this file type.
+    UNKNOWN = 5;
   }
   required Verdict verdict = 1;
 
   message MoreInfo {
     // A human-readable string describing the nature of the warning.
     // Only if verdict != SAFE. Localized based on request.locale.
     optional string description = 1;
 
     // A URL to get more information about this warning, if available.
     optional string url = 2;
@@ skipping 379 matching lines @@
   optional bool did_proceed = 8;
 
   // Whether user visited this origin before.
   optional bool repeat_visit = 9;
 
   // The same token in ClientDownloadResponse. This field is only set if its
   // report type is DANGEROUS_DOWNLOAD_RECOVERY, DANGEROUS_DOWNLOAD_WARNING or
   // DANGEROUS_DOWNLOAD_BY_API.
   optional bytes token = 15;
 }