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

chrome/browser/sync/engine/model_safe_worker.h

Index: chrome/browser/sync/engine/model_safe_worker.h
diff --git a/chrome/browser/sync/engine/model_safe_worker.h b/chrome/browser/sync/engine/model_safe_worker.h
index f4829ec4b58140f9d37d57541ff0bb7ac4d25c38..ed74589d46bea2ff70ec7df50bb85615f4d20ca2 100644
--- a/chrome/browser/sync/engine/model_safe_worker.h
+++ b/chrome/browser/sync/engine/model_safe_worker.h
@@ -10,9 +10,11 @@
 #include <string>
 #include <vector>
 
+#include "base/callback.h"
 #include "base/callback_old.h"
 #include "base/memory/ref_counted.h"
 #include "chrome/browser/sync/syncable/model_type.h"
+#include "chrome/browser/sync/sessions/unrecoverable_error_info.h"
 
 namespace base {
 class DictionaryValue;
@@ -20,6 +22,8 @@ class DictionaryValue;
 
 namespace browser_sync {
 
+typedef base::Callback<sessions::UnrecoverableErrorInfo(void)> WorkCallback;
+
 enum ModelSafeGroup {
   GROUP_PASSIVE = 0,   // Models that are just "passively" being synced; e.g.
                        // changes to these models don't need to be pushed to a
@@ -52,7 +56,8 @@ class ModelSafeWorker : public base::RefCountedThreadSafe<ModelSafeWorker> {
   // Any time the Syncer performs model modifications (e.g employing a
   // WriteTransaction), it should be done by this method to ensure it is done
   // from a model-safe thread.
-  virtual void DoWorkAndWaitUntilDone(Callback0::Type* work);
+  virtual sessions::UnrecoverableErrorInfo DoWorkAndWaitUntilDone(
+      const WorkCallback& work);
 
   virtual ModelSafeGroup GetModelSafeGroup();