Send the referral URL with the client-side phishing detection request.

chrome/common/safe_browsing/csd.proto

 // 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.
 //
 // Client side phishing and malware detection request and response
 // protocol buffers.  Those protocol messages should be kept in sync
 // with the server implementation.
 //
 // If you want to change this protocol definition or you have questions
 // regarding its format please contact chrome-anti-phishing@googlegroups.com.
@@ skipping 35 matching lines @@
 
   // The version number of the model that was used to compute the client-score.
   // Copied from ClientSideModel.version().
   optional int32 model_version = 6;
 
   // Field 7 is only used on the server.
 
   // List of features that are extracted in the client but are not used in the
   // machine learning model.
   repeated Feature non_model_feature_map = 8;
+
+  // The referrer URL.  This field might not be set, for example, in the case
+  // where the referrer uses HTTPs.
+  optional string referrer_url = 9;
 }
 
 message ClientPhishingResponse {
   required bool phishy = 1;
 }