Implement exponential back-off mechanism and enforce it at the URLRequestHttpJob level.

net/base/net_error_list.h

 // Copyright (c) 2010 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 file intentionally does not have header guards, it's included
 // inside a macro to generate enum.
 
 // This file contains the list of network errors.
 
 //
@@ skipping 192 matching lines @@
 NET_ERROR(PROXY_CERTIFICATE_INVALID, -136)
 
 // An error occurred when trying to do a name resolution (DNS).
 NET_ERROR(NAME_RESOLUTION_FAILED, -137)
 
 // Permission to access the network was denied. This is used to distinguish
 // errors that were most likely caused by a firewall from other access denied
 // errors. See also ERR_ACCESS_DENIED.
 NET_ERROR(NETWORK_ACCESS_DENIED, -138)
 
+// The request throttler module cancelled this request to avoid DDOS.
+NET_ERROR(TEMPORARILY_THROTTLED, -139)
+
 // Certificate error codes
 //
 // The values of certificate error codes must be consecutive.
 
 // The server responded with a certificate whose common name did not match
 // the host name.  This could mean:
 //
 // 1. An attacker has redirected our traffic to his server and is
 //    presenting a certificate for which he knows the private key.
 //
@@ skipping 276 matching lines @@
 // Import failed - certificate already exists in database.
 // Note it's a little weird this is an error but reimporting a PKCS12 is ok
 // (no-op).  That's how Mozilla does it, though.
 NET_ERROR(IMPORT_CERT_ALREADY_EXISTS, -704)
 
 // CA import failed due to some other error.
 NET_ERROR(IMPORT_CA_CERT_FAILED, -705)
 
 // Server certificate import failed due to some internal error.
 NET_ERROR(IMPORT_SERVER_CERT_FAILED, -706)