Generalize the usage of referrer chain

components/safe_browsing/csd.proto

 // 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.
 //
 // This proto file includes:
 // (1) Client side phishing and malware detection request and response
 //   protocol buffers.  Those protocol messages should be kept in sync
 //   with the server implementation.
 //
 // (2) Safe Browsing reporting protocol buffers.
@@ skipping 488 matching lines @@
   // order, i.e. download url comes first in this list, and landing referrer
   // comes last.
   repeated ReferrerChainEntry referrer_chain = 36;
 
   // Whether DownloadAttribution Finch experiment is enabled for this ping.
   optional bool download_attribution_finch_enabled = 39;
 }
 
 message ReferrerChainEntry {
   enum URLType {
-    DOWNLOAD_URL = 1;
+    // URL of safe browsing events that are at the end of the referrer chain.
+    // e.g. URL of a download, URL of a low reputation login page, etc.
+    EVENT_URL = 1;  // e.g.
+
+    // Landing page is the page user directly interacts with to trigger the
+    // above event, e.g. the page where user clicks a download button.
     LANDING_PAGE = 2;
+
+    // Landing referrer is the one user directly interacts with right before
+    // navigating to the landing page.
     LANDING_REFERRER = 3;
+
+    // Client redirect refers to committed navigation between landing page and
+    // the targeted event, or between landing referrer page and landing page.
+    // Client redirect is not triggered by user gesture.
     CLIENT_REDIRECT = 4;
+
     DEPRECATED_SERVER_REDIRECT = 5;  // Deprecated
   }
 
   message ServerRedirect {
     // [required] server redirect url
     optional string url = 1;
 
     // Additional fields for future expansion.
   }
 
   // [required] The url of this Entry.
   optional string url = 1;
 
   // Only set if it is different from |url|.
   optional string main_frame_url = 9;
 
-  // Type of URLs, such as download url, download referrer, etc.
+  // Type of URLs, such as event url, landing page, etc.
   optional URLType type = 2 [default = CLIENT_REDIRECT];
 
   // IP addresses corresponding to this host.
   repeated string ip_addresses = 3;
 
   // Referrer url of this entry.
   optional string referrer_url = 4;
 
   // Main frame URL of referrer.
   // Only set if it is different from |referrer_url|.
@@ skipping 481 matching lines @@
 // There is no response (an empty body) to this request.
 message NotificationImageReportRequest {
   optional string notification_origin = 1;  // Src-origin of the notification.
   optional ImageData image = 2;             // The bitmap of the image.
 
   // Note that the image URL is deliberately omitted as it would be untrusted,
   // since the notification image fetch may be intercepted by a Service Worker
   // (even if the image URL is cross-origin). Otherwise a website could mislead
   // Safe Browsing into associating phishing image bitmaps with safe image URLs.
 }