[Sync] Remove backend unrecoverable error handler

sync/util/mock_unrecoverable_error_handler.h

Index: sync/util/mock_unrecoverable_error_handler.h
diff --git a/sync/util/mock_unrecoverable_error_handler.h b/sync/util/mock_unrecoverable_error_handler.h
index aacde89e852fb879ce5d1f9f3021a5526b1ca32f..0d80fe9251d37567754c2d9a3c7837b44094478c 100644
--- a/sync/util/mock_unrecoverable_error_handler.h
+++ b/sync/util/mock_unrecoverable_error_handler.h
@@ -8,6 +8,7 @@
 #include <string>
 
 #include "base/macros.h"
+#include "base/memory/weak_ptr.h"
 #include "sync/internal_api/public/util/unrecoverable_error_handler.h"
 
 namespace syncer {
@@ -24,9 +25,13 @@ class MockUnrecoverableErrorHandler : public UnrecoverableErrorHandler {
   // 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);
 };