RIYDAH, June 8 (Reuters) - Gunmen shot dead an American who worked for a U.S. contracting firm in the Saudi capital Riyadh on Tuesday, the fifth assault on Westerners in the world's leading oil exporter in five weeks. The latest killing in the kingdom, battling militants loyal to Saudi-born Osama bin Laden's al Qaeda network, heightened fears among tens of thousands of expatriates, many of whom work in the oil industry. In Washington, a U.S. State Department official confirmed that an American man was killed in Riyadh but said he could not provide details, pending notification of the family. "I...