Two low-profile Texas brothers have donated $15 million to support Sen. Ted Cruz, a record-setting contribution that amounts to the largest known donation so far in the 2016 presidential campaign. Farris and Dan Wilks, billionaires who made their fortunes in the West Texas fracking boom, have given $15 million of the $38 million that the pro-Cruz super PAC, Keep the Promise, will disclose in election filings next week, according to sources outside the super PAC with knowledge of the giving. The siblings earned their riches with the sale of their company Frac Tech for $3.5 billion in 2011, and since...