Hong Kong riot police fired tear gas and water cannons at hardcore pro-democracy protesters hurling rocks and petrol bombs on Sunday, tipping the violence-plagued city back into chaos after a brief lull in clashes.Tens of thousands of people defied authorities to march through the city in an unsanctioned rally on Sunday, the latest expression of a popular revolt that has raged for the last 99 days.The rally descended into violence when small groups of hardcore activists – known within the mov