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..9421e7213e88cd101a6186744aa7cbdc04f03569 100644
--- a/chrome/browser/android/webapk/webapk.proto
+++ b/chrome/browser/android/webapk/webapk.proto
@@ -26,7 +26,14 @@ message WebApkResponse {
   // 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.

[pkotwicz, 2017/03/04 00:20:47]

This is not really correct. The client can determine whether the WebAPK needs an
update. However, sometimes the server is not able to fulfill an update request
for technical reasons

How about: "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 ..."

[Xi Han, 2017/03/06 22:14:21]

Makes sense, thanks for the better description.

+  optional bool relax_updates = 8;
+
+  reserved 4, 5, 7;
 }
 
 // Sent as part of request to create or update a WebAPK.