To challenge the power of his chief rival, the United States, China’s top leader, Xi Jinping, has linked arms with two anti-Western states, declaring a “no limits” partnership with Russia and pledging “unswerving” support for North Korea. But the specter of a budding bromance between President Vladimir V. Putin of Russia and the North Korean leader, Kim Jong-un, after their meeting this week in eastern Russia, may not be as welcome a development for Mr. Xi as it might initially seem. Closer ties between Pyongyang and Moscow could result in…