(Adds external expert comment after report published) By Christopher Bing Feb 28 (Reuters) - Security researchers with U.S. cybersecurity firm Symantec said they have discovered a “highly sophisticated” Chinese hacking tool that has been able to escape public attention for more than a decade. The discovery was shared with the U.S. government in recent months, who have shared the information with foreign partners, said a U.S. official. Symantec, a division of chipmaker Broadcom , published its research about the tool, which it calls Daxin, on Monday. “It’s something we haven’t seen before,” said Clayton Romans, associate director with the U.S....