Server directed error handling backend code.

chrome/browser/sync/protocol/sync.proto

Index: chrome/browser/sync/protocol/sync.proto
diff --git a/chrome/browser/sync/protocol/sync.proto b/chrome/browser/sync/protocol/sync.proto
index ae118ce60de42330947759dc65cfe1335839db41..cb4be4fd09d7ec5b6c203a2cec981c74051f03be 100644
--- a/chrome/browser/sync/protocol/sync.proto
+++ b/chrome/browser/sync/protocol/sync.proto
@@ -627,13 +627,34 @@ message ClientToServerResponse {
                               // out-of-date client parses a value it doesn't
                               // recognize.
   }
+
+  message Error {
+    optional ErrorType error_type       = 1 [default = UNKNOWN];
+    optional string error_description   = 2;
+    optional string url                 = 3;
+    enum Action {
+      UPGRADE_CLIENT              = 0; // Upgrade the client to latest version.
+      CLEAR_USER_DATA_AND_RESYNC  = 1; // Clear user data from dashboard and
+                                       // setup sync again.
+      ENABLE_SYNC_ON_ACCOUNT      = 2; // The administrator needs to enable sync
+                                       // on the account.
+      STOP_AND_RESTART_SYNC       = 3; // Stop sync and set up sync again.
+      DISABLE_SYNC_ON_CLIENT      = 4; // Wipe the client of all sync data and
+                                       // stop syncing.
+      UNKNOWN_ACTION              = 5; // This is the default.
+    }
+    optional Action action              = 4 [default = UNKNOWN_ACTION];
+  }
+
+  optional Error error = 13;
+
   // Up until protocol_version 24, the default was SUCCESS which made it
   // impossible to add new enum values since older clients would parse any
   // out-of-range value as SUCCESS. Starting with 25, unless explicitly set,
   // the error_code will be UNKNOWN so that clients know when they're
   // out-of-date. Note also that when using protocol_version < 25,
   // TRANSIENT_ERROR is not supported. Instead, the server sends back a HTTP
-  // 400 error code.
+  // 400 error code. This is deprecated now.
   optional ErrorType error_code = 4 [default = UNKNOWN];
   optional string error_message = 5;