By Martin Quin Pollard GUANGZHOU, China (Reuters) -A China Eastern Airlines Boeing 737-800 with 132 people on board crashed in mountains in southern China on a domestic flight on Monday after a sudden descent from cruising altitude. Media said there were no signs of survivors. The airline said it deeply mourned the passengers and crew, without specifying how many people had been killed. Boeing said it was ready to assist China Eastern and was in contact with U.S. transportation safety regulators over the incident. Chinese media carried brief highway video footage from a vehicle’s dashcam appar…