Don't download identical APKs on update.

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.
+  // 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;
 }
 
 // Sent as part of request to create or update a WebAPK.
 message WebApk {
@@ skipping 56 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;
 }