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

chrome/browser/sync/engine/model_changing_syncer_command.h

Index: chrome/browser/sync/engine/model_changing_syncer_command.h
diff --git a/chrome/browser/sync/engine/model_changing_syncer_command.h b/chrome/browser/sync/engine/model_changing_syncer_command.h
index f6d6a436ac7e22966609258a08681b3f0a7df171..3a43cdca204ba9812eab1e875929120fbff3ab86 100644
--- a/chrome/browser/sync/engine/model_changing_syncer_command.h
+++ b/chrome/browser/sync/engine/model_changing_syncer_command.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include "chrome/browser/sync/engine/syncer_command.h"
+#include "chrome/browser/sync/sessions/unrecoverable_error_info.h"
 
 namespace browser_sync {
 namespace sessions {
@@ -32,8 +33,11 @@ class ModelChangingSyncerCommand : public SyncerCommand {
   virtual void ExecuteImpl(sessions::SyncSession* session);
 
   // wrapper so implementations don't worry about storing work_session
-  void StartChangingModel() {
+  sessions::UnrecoverableErrorInfo StartChangingModel() {
+    // TODO(lipalani): |ModelChangingExecuteImpl| should return an
+    // UnrecoverableErrorInfo struct.
     ModelChangingExecuteImpl(work_session_);
+    return sessions::UnrecoverableErrorInfo();
   }
 
   // Sometimes, a command has work to do that needs to touch global state