Introduce the plumbing necessary to report Unrecoverable error from model safe workers.

chrome/browser/sync/engine/apply_updates_command.cc

Index: chrome/browser/sync/engine/apply_updates_command.cc
diff --git a/chrome/browser/sync/engine/apply_updates_command.cc b/chrome/browser/sync/engine/apply_updates_command.cc
index 6d1f9583da1db07b210a81b2bc5231cbeba0a035..ba7eed8fe141ae6455f26fb6be6f6768a569fa40 100644
--- a/chrome/browser/sync/engine/apply_updates_command.cc
+++ b/chrome/browser/sync/engine/apply_updates_command.cc
@@ -34,6 +34,18 @@ void ApplyUpdatesCommand::ModelChangingExecuteImpl(SyncSession* session) {
       handles.begin(), handles.end(), session->routing_info(),
       session->status_controller()->group_restriction());
   while (applicator.AttemptOneApplication(&trans)) {}
+
+  if (applicator.failed_update()) {
+    // TODO(lipalani): Pipe in the failed line number from the syncable layer.
+
+    // We could be in any thread here. So set the unrecoverable error in.
+    // session. ModelChangingSyncerCommand object will send it on the right
+    // thread.
+    session->SetUnrecoverableError(FROM_HERE, "Failed to apply update.",
+        &trans);
+    return;
+  }
+
   applicator.SaveProgressIntoSessionState(
       session->status_controller()->mutable_conflict_progress(),
       session->status_controller()->mutable_update_progress());