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. Note that this can be empty, and if it is, 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 WAM 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 already 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.
+  optional bool infrequent_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 41 matching lines @@
   // Murmur2 hash of the icon's bytes. There should not be any transformations
   // applied to the icon's bytes prior to taking the Murmur2 hash.
   optional string hash = 5;
 
   // 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;
 
   reserved 2 to 4;
 }