[Downloads] Gracefully handle SafeBrowsing check failures.

chrome/browser/safe_browsing/download_protection_service.h

 // 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.
 //
 // Helper class which handles communication with the SafeBrowsing servers for
 // improved binary download protection.
 
 #ifndef CHROME_BROWSER_SAFE_BROWSING_DOWNLOAD_PROTECTION_SERVICE_H_
 #define CHROME_BROWSER_SAFE_BROWSING_DOWNLOAD_PROTECTION_SERVICE_H_
 
@@ skipping 23 matching lines @@
 
 namespace safe_browsing {
 class DownloadFeedbackService;
 class BinaryFeatureExtractor;
 
 // This class provides an asynchronous API to check whether a particular
 // client download is malicious or not.
 class DownloadProtectionService {
  public:
   enum DownloadCheckResult {
+    UNKNOWN,
     SAFE,
     DANGEROUS,
     UNCOMMON,
     DANGEROUS_HOST,
     POTENTIALLY_UNWANTED
   };
 
   // Callback type which is invoked once the download request is done.
   typedef base::Callback<void(DownloadCheckResult)> CheckDownloadCallback;
 
@@ skipping 141 matching lines @@
 
   int64 download_request_timeout_ms_;
 
   scoped_ptr<DownloadFeedbackService> feedback_service_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadProtectionService);
 };
 }  // namespace safe_browsing
 
 #endif  // CHROME_BROWSER_SAFE_BROWSING_DOWNLOAD_PROTECTION_SERVICE_H_