Throughout President Donald Trump’s political career, Rupert Murdoch has played the role of a scold and a cheerleader. He has been a target, and frequent beneficiary, of Trump’s whims. But Trump’s recent lawsuit against the Murdoch-owned Wall Street Journal tests their mutually beneficial bond. On Friday, one day after the Journal published a story alleging that Trump wrote a “bawdy” birthday letter to financier and deceased sexual predator Jeffrey Epstein, Trump sued the Journal, the two authors of the story and a raft of corporate overseers including Murdoch, whose family trust controls the Journal’s parent company and that of its...