[Sync] Remove backend unrecoverable error handler

sync/util/mock_unrecoverable_error_handler.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef SYNC_UTIL_MOCK_UNRECOVERABLE_ERROR_HANDLER_H_
 #define SYNC_UTIL_MOCK_UNRECOVERABLE_ERROR_HANDLER_H_
 
 #include <string>
 
 #include "base/macros.h"
+#include "base/memory/weak_ptr.h"
 #include "sync/internal_api/public/util/unrecoverable_error_handler.h"
 
 namespace syncer {
 
 // Mock implementation of UnrecoverableErrorHandler that counts how many times
 // it has been invoked.
 class MockUnrecoverableErrorHandler : public UnrecoverableErrorHandler {
  public:
   MockUnrecoverableErrorHandler();
   ~MockUnrecoverableErrorHandler() override;
   void OnUnrecoverableError(const tracked_objects::Location& from_here,
                             const std::string& message) override;
 
   // Returns the number of times this handler has been invoked.
   int invocation_count() const;
 
+  base::WeakPtr<MockUnrecoverableErrorHandler> GetWeakPtr();
+
  private:
   int invocation_count_;
 
+  base::WeakPtrFactory<MockUnrecoverableErrorHandler> weak_ptr_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(MockUnrecoverableErrorHandler);
 };
 
 }  // namespace syncer
 
 #endif  // SYNC_UTIL_MOCK_UNRECOVERABLE_ERROR_HANDLER_H_