Chrome talks to Play to install WebAPKs.

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.
   optional string signed_download_url = 3;
 
-  reserved 4;
+  // Unique id identifying session with WebAPK server.
+  optional string wam_token = 6;

[pkotwicz, 2016/12/02 02:24:00]

I like "token" better than "wam_token". People outside our team don't know what
the "wam" acronym stands for.

[Xi Han, 2016/12/02 20:45:37]

All right, change it back.

+
+  reserved 4, 5;
 }
 
 // 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 37 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;
 }