The Pentagon on Saturday said that Beijing had agreed to return an underwater drone seized by China in international waters, an indication that the two countries were moving to resolve an unusual incident that risked sharpening tensions in the run-up to the inauguration of President-elect Donald J. Trump. “Through direct engagement with Chinese authorities, we have secured an understanding that the Chinese will return the U.U.V. to the United States,” said Peter Cook, the Pentagon press secretary, using initials to refer to the Navy’s unmanned underwater vehicle.