Only enable improved SafeBrowsing download protection on Windows.

chrome/browser/safe_browsing/download_protection_service.h

 // 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.
 //
 // 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_
 #pragma once
@@ skipping 107 matching lines @@
     REASON_SERVER_PING_FAILED,
     REASON_INVALID_RESPONSE_PROTO,
     REASON_NOT_BINARY_FILE,
     REASON_REQUEST_CANCELED,
     REASON_DOWNLOAD_DANGEROUS,
     REASON_DOWNLOAD_SAFE,
     REASON_EMPTY_URL_CHAIN,
     REASON_HTTPS_URL,
     REASON_PING_DISABLED,
     REASON_TRUSTED_EXECUTABLE,
+    REASON_OS_NOT_SUPPORTED,
     REASON_MAX  // Always add new values before this one.
   };
 
  private:
   class CheckClientDownloadRequest;  // Per-request state
   friend class DownloadProtectionServiceTest;
   FRIEND_TEST_ALL_PREFIXES(DownloadProtectionServiceTest,
                            CheckClientDownloadValidateRequest);
   FRIEND_TEST_ALL_PREFIXES(DownloadProtectionServiceTest,
                            CheckClientDownloadSuccess);
@@ skipping 33 matching lines @@
   // SignatureUtil object, may be overridden for testing.
   scoped_refptr<SignatureUtil> signature_util_;
 
   int64 download_request_timeout_ms_;
 
   DISALLOW_COPY_AND_ASSIGN(DownloadProtectionService);
 };
 }  // namespace safe_browsing
 
 #endif  // CHROME_BROWSER_SAFE_BROWSING_DOWNLOAD_PROTECTION_SERVICE_H_