[Sync] Remove backend unrecoverable error handler

sync/util/mock_unrecoverable_error_handler.cc

 // 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.
 
 #include "sync/util/mock_unrecoverable_error_handler.h"
 
 namespace syncer {
 
 MockUnrecoverableErrorHandler::MockUnrecoverableErrorHandler()
-    : invocation_count_(0) {
+    : invocation_count_(0),
+      weak_ptr_factory_(this) {
 }
 
 MockUnrecoverableErrorHandler::~MockUnrecoverableErrorHandler() {
 }
 
 void MockUnrecoverableErrorHandler::OnUnrecoverableError(
     const tracked_objects::Location& from_here,
     const std::string& message) {
   ++invocation_count_;
 }
 
 int MockUnrecoverableErrorHandler::invocation_count() const {
   return invocation_count_;
 }
 
+base::WeakPtr<MockUnrecoverableErrorHandler>
+MockUnrecoverableErrorHandler::GetWeakPtr() {
+  return weak_ptr_factory_.GetWeakPtr();
+}
+
 }  // namespace syncer