Add iOS variable for --host-resolver-rules switch.

ios/chrome/browser/chrome_switches.cc

 // Copyright 2015 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.
 
 #include "ios/chrome/browser/chrome_switches.h"
 
 namespace switches {
 
 // -----------------------------------------------------------------------------
 // When commenting your switch, please use the same voice as surrounding
@@ skipping 73 matching lines @@
 // Enables the recording of metrics reports but disables reporting. In contrast
 // to kDisableMetrics, this executes all the code that a normal client would
 // use for reporting, except the report is dropped rather than sent to the
 // server. This is useful for finding issues in the metrics code during UI and
 // performance tests.
 const char kIOSMetricsRecordingOnly[] = "metrics-recording-only";
 
 // A string used to override the default user agent with a custom one.
 const char kUserAgent[] = "user-agent";
 
+// These mappings only apply to the host resolver.
+const char kIOSHostResolverRules[] = "host-resolver-rules";
+
 }  // namespace switches