Fix unittest build.

The recent change to return the last error code broke the unittest
build.

Bug: 25598547
TEST=FEATURES=test emerge-link update_engine

Change-Id: I8fce1ded54a7a941cb3ac1d6a150217a8994bf5a
diff --git a/mock_payload_state.h b/mock_payload_state.h
index 728f274..2f654c7 100644
--- a/mock_payload_state.h
+++ b/mock_payload_state.h
@@ -74,6 +74,7 @@
   MOCK_CONST_METHOD0(GetUsingP2PForSharing, bool());
   MOCK_METHOD0(GetScatteringWaitPeriod, base::TimeDelta());
   MOCK_CONST_METHOD0(GetP2PUrl, std::string());
+  MOCK_CONST_METHOD0(GetAttemptErrorCode, ErrorCode());
 };
 
 }  // namespace chromeos_update_engine