Xi Jinping, China’s leader, is known as the Chairman of Everything. He makes decisions daily on the economy, the military, foreign policy, human rights and more. Yet on North Korea he is stuck. A strongman who usually acts with precision and boldness, Mr. Xi has been reluctant to take on the North’s leader, Kim Jong-un, ostensibly a Chinese ally, whom he privately disparages to Western leaders as young and reckless. The July 4 test of the North’s first intercontinental ballistic missile has raised the question of what is China’s red line for its ally, and whether the test will force Mr. Xi to act decisively against North Korea as the Trump administration is asking him to do.