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

chrome/browser/sync/engine/model_safe_worker.cc

Index: chrome/browser/sync/engine/model_safe_worker.cc
diff --git a/chrome/browser/sync/engine/model_safe_worker.cc b/chrome/browser/sync/engine/model_safe_worker.cc
index ae94b265de3361ae000af9318337e9f48ee29d0d..3da1abb5fd9120b97d355883af6e6dcc0aa9e64c 100644
--- a/chrome/browser/sync/engine/model_safe_worker.cc
+++ b/chrome/browser/sync/engine/model_safe_worker.cc
@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include "chrome/browser/sync/engine/model_safe_worker.h"
+#include "chrome/browser/sync/sessions/unrecoverable_error_info.h"
 
 #include "base/json/json_writer.h"
 #include "base/memory/scoped_ptr.h"
@@ -10,6 +11,8 @@
 
 namespace browser_sync {
 
+using sessions::UnrecoverableErrorInfo;
+
 base::DictionaryValue* ModelSafeRoutingInfoToValue(
     const ModelSafeRoutingInfo& routing_info) {
   base::DictionaryValue* dict = new base::DictionaryValue();
@@ -74,8 +77,10 @@ ModelSafeWorker::ModelSafeWorker() {}
 
 ModelSafeWorker::~ModelSafeWorker() {}
 
-void ModelSafeWorker::DoWorkAndWaitUntilDone(Callback0::Type* work) {
-  work->Run();  // For GROUP_PASSIVE, we do the work on the current thread.
+UnrecoverableErrorInfo ModelSafeWorker::DoWorkAndWaitUntilDone(
+  const WorkCallback& work) {
+  // For GROUP_PASSIVE, we do the work on the current thread.
+  return work.Run();
 }
 
 ModelSafeGroup ModelSafeWorker::GetModelSafeGroup() {