HONG KONG—Hong Kong has convicted a second person under its sweeping national security law for chanting pro-independence slogans, amid a political clampdown in the city. Ma Chun-man was convicted of inciting secession on Monday after he was found to have chanted slogans such as “Hong Kong independence, the only way out” on 20 occasions between August and November of last year. Critics in Hong Kong say the national security law erodes freedoms, such as those of expression and assembly, that were promised to the city for 50 years when the former British colony was handed over to China in 1997. Ma is the second person to be convicted under the new law, which outlaws what authorities describe as secession, subversion of state power, terrorism, and foreign collusion in intervening in Hong Kong’s affairs. Ma had been arrested multiple times for chanting such slogans before being remanded in custody. Sentencing for …