native_backend_test.cc: remove using CheckThreadSafety
native_backend_test.cc: remove using CheckThreadSafety

niboshi/chainer,keisuke-umezawa/chainer,okuta/chainer,niboshi/chainer,wkentaro/chainer,jnishi/chainer,hvy/chainer,pfnet/chainer,wkentaro/chainer,chainer/chainer,ktnyt/chainer,niboshi/chainer,keisuke-umezawa/chainer,tkerola/chainer,wkentaro/chainer,jnishi/chainer,jnishi/chainer,ktnyt/chainer,hvy/chainer,ktnyt/chainer,chainer/chainer,jnishi/chainer,okuta/chainer,okuta/chainer,ktnyt/chainer,chainer/chainer,hvy/chainer,keisuke-umezawa/chainer,okuta/chainer,hvy/chainer,niboshi/chainer,wkentaro/chainer,keisuke-umezawa/chainer,chainer/chainer