Implement server-suggested update check backoff

chrome/browser/android/webapk/webapk.proto

 // Copyright 2016 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.
 
 syntax = "proto2";
 
 option optimize_for = LITE_RUNTIME;
 
 package webapk;
 
 // Response after creating or updating a WebAPK.
 message WebApkResponse {
   // Package name to install WebAPK at.
   optional string package_name = 1;
 
   // Version code of the WebAPK.
   optional string version = 2;
 
   // URL to download WebAPK. If the URL is empty, the client should not attempt
   // to download or install anything. For example, this may be empty when the
   // client requests an update even though no content has changed. The WebAPK
   // server can then let the client know that it is up-to-date already and does
   // not need to spend bandwidth re-downloading a WebAPK it has.
   optional string signed_download_url = 3;
 
   // Unique id identifying session with WebAPK server.
   optional string token = 6;
 
-  reserved 4, 5;
+  // If true, this directs the client to check for updates less frequently than
+  // normal. This flag may be used, for example, if the WAM server knows that
+  // the client has an up-to-date WebAPK, but the client is unable to determine
+  // that for itself. Therefore, we want the client to back off from spamming
+  // update requests too often.

[pkotwicz, 2017/03/04 00:20:47]

This is not really correct. The client can determine whether the WebAPK needs an
update. However, sometimes the server is not able to fulfill an update request
for technical reasons

How about: "This flag may be used, for example, if the WebAPK server is unable
to fulfill an update request and likely won't be able to fulfill subsequent
update requests from this WebAPK. One case where the server is unable to fulfill
update requests is if the Web Manifest is dynamically generated based on the
user's country etc. Therefore, we want the client ..."

[Xi Han, 2017/03/06 22:14:21]

Makes sense, thanks for the better description.

+  optional bool relax_updates = 8;
+
+  reserved 4, 5, 7;
 }
 
 // Sent as part of request to create or update a WebAPK.
 message WebApk {
   // Package name of the WebAPK.
   optional string package_name = 1;
 
   // Version code of the WebAPK.
   optional string version = 2;
 
@@ skipping 50 matching lines @@
 
   // Actual bytes of the image. This image may be re-encoded from the original
   // image and may not match the murmur2 hash field above.
   optional bytes image_data = 6;
 
   // Specifies Chrome's intended usages for the image.
   repeated Usage usages = 8;
 
   reserved 2, 3, 4, 7;
 }