[Sync] Update ChipBag proto

Add an optional bytes server_chips field to represent the opaque server data.

BUG=None


Review URL: https://chromiumcodereview.appspot.com/11361234

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@167586 0039d316-1c4b-4281-b951-d872f2087c98

sync/protocol/sync.proto

@@ -469,9 +469,24 @@ message AuthenticateMessage {
   required string auth_token = 1;
 };
 
-// Opaque data used by the server to track the client with.
+// The client must preserve, store, and resend the chip bag with
+// every request.  The server depends on the chip bag in order
+// to precisely choreograph a client-server state machines.
+//
+// Because the client stores and sends this data on every request,
+// the contents of the chip bag should be kept relatively small.
+//
+// If the server does not return a chip bag, the client must assume
+// that there has been no change to the chip bag.  The client must
+// resend the bag of chips it had prior on the next request.
+//
+// The client must make the chip bag durable if and only if it
+// processes the response from the server.
 message ChipBag {
-};
+  // Server chips are deliberately oqaque, allowing the server
+  // to encapsulate its state machine logic.
+  optional bytes server_chips = 1;
+}
 
 // Information about the syncer's state.
 message ClientStatus {