In taking the unusual step of approving an old generic drug as a treatment for autism, the Food and Drug Administration stunned some experts by departing sharply from the agency’s typical standard for reviewing drugs. The drug, leucovorin, has long been used to treat the toxic effects of chemotherapy, but it was endorsed as a therapy for some people with autism by President Trump and top U.S. health officials during a White House briefing on Monday. The move flipped the standard process: Typically, a pharmaceutical company carefully studies a drug, often with input from the F.D.A. on the design of rigorous studies, and then files a formal application for approval. But in this case, the agency said it reviewed medical research and made the approval decision to expand the drug’s use on its own.