A World Trade Organization arbitrator will step into a years-old dispute brought by China over US anti-dumping measures.  A trade official said the arbitration was triggered automatically after the US objected to Beijing's request for authorization to retaliate against more than $7B worth of US goods in the case.  The case originated with a Chinese challenge nearly 5 years ago over 40 US anti-dumping rulings against Chinese goods, which the US says were sold below market value.  In 2017, the WTO's appellate body ruled largely in favor of China.  Beijing insists the US has not complied.  The arbitrator has until Oct 21 to rule on the issue, but such cases often go past the deadline.  The standoff comes amid a high-profile showdown between China & the Trump administration over trade.