Chrome talks to Play to install WebAPKs.

chrome/browser/android/webapk/webapk.proto

Index: chrome/browser/android/webapk/webapk.proto
diff --git a/chrome/browser/android/webapk/webapk.proto b/chrome/browser/android/webapk/webapk.proto
index 4a18b4cce14806409753ac880915a3d9dbc0a375..fc0cae7b54ca31d644359bac854e7141d64aa5d7 100644
--- a/chrome/browser/android/webapk/webapk.proto
+++ b/chrome/browser/android/webapk/webapk.proto
@@ -19,7 +19,10 @@ message WebApkResponse {
   // URL to download WebAPK.
   optional string signed_download_url = 3;
 
-  reserved 4;
+  // The token to give the Phonesky backend for the rest of the information.

[pkotwicz, 2016/11/23 03:11:28]

How about: "Unique id identifying session with WebAPK server."

Maybe Scott has ideas for naming/documenting the proto member.
I am not a big fan of the "token" name

[Xi Han, 2016/11/23 15:43:45]

Updated.
 
The webapk.proto declared on the server side uses "token", and Play also have
"token" as a key word in the installOptions. Personally, I would prefer to use
"token" to avoid renaming on all these places.

[pkotwicz, 2016/11/23 15:56:38]

Let's rename it in Chrome so that at least Chrome uses a more meaningful name
than "token"

[Xi Han, 2016/11/23 16:44:42]

How about "wam_token" as the one sent to Play?

+  optional string token = 6;
+
+  reserved 4, 5;
 }
 
 // Sent as part of request to create or update a WebAPK.