[Sync] Remove backend unrecoverable error handler

sync/util/mock_unrecoverable_error_handler.cc

Index: sync/util/mock_unrecoverable_error_handler.cc
diff --git a/sync/util/mock_unrecoverable_error_handler.cc b/sync/util/mock_unrecoverable_error_handler.cc
index bfed1f1e0cbabed30c4fe86a4cab24eaec8544ae..3f34eca681bbd99da5e0694822145321652e84a4 100644
--- a/sync/util/mock_unrecoverable_error_handler.cc
+++ b/sync/util/mock_unrecoverable_error_handler.cc
@@ -7,7 +7,8 @@
 namespace syncer {
 
 MockUnrecoverableErrorHandler::MockUnrecoverableErrorHandler()
-    : invocation_count_(0) {
+    : invocation_count_(0),
+      weak_ptr_factory_(this) {
 }
 
 MockUnrecoverableErrorHandler::~MockUnrecoverableErrorHandler() {
@@ -23,4 +24,9 @@ int MockUnrecoverableErrorHandler::invocation_count() const {
   return invocation_count_;
 }
 
+base::WeakPtr<MockUnrecoverableErrorHandler>
+MockUnrecoverableErrorHandler::GetWeakPtr() {
+  return weak_ptr_factory_.GetWeakPtr();
+}
+
 }  // namespace syncer