Exxon Mobil Corp. on Wednesday outlined an ambitious plan to turn to prolific U.S. shale basins for growth, showcasing how the oil giant now sees American production as a key to its future. The company plans to spend about a fourth of its 2017 budget—about $5.5 billion—drilling in Texas, New Mexico and North Dakota, tapping a vast inventory of wells that can turn a profit at a price of $40 a barrel. The U.S. increasingly…