Cast size_t type to int type explicitly
Cast size_t type to int type explicitly

Currently, the generated header file might cause warnings if user specifies `-Wconversion` compiler option. This commit should be able to fix it.
google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf