Implement server-suggested update check backoff

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 27fc96b0842a89ebad031e709b97ba27c5b6126d..614d514383733e1cccf7f5fc334142645d83a0e9 100644
--- a/chrome/browser/android/webapk/webapk.proto
+++ b/chrome/browser/android/webapk/webapk.proto
@@ -26,7 +26,15 @@ message WebApkResponse {
   // Unique id identifying session with WebAPK server.
   optional string token = 6;
 
-  reserved 4, 5;
+  // 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 to back off
+  // from spamming update requests too often.
+  optional bool relax_updates = 8;
+
+  reserved 4, 5, 7;
 }
 
 // Sent as part of request to create or update a WebAPK.