Over this past offseason, the 49ers pursued and signed former Raiders standout cornerback Nnamdi Asomugha in an effort to upgrade their pass coverage capabilities.But five games into the 2013 season, it appears the player most capable of doing just that has been on the San Francisco roster all along.Tramaine Brock, signed by the 49ers in 2010 as an undrafted free agent out of little NAIA school Belhaven University, took over the No. 3 cornerback job from the injured Asomugha in Game 4 vs. the Rams, then had two interceptions – including one he returned for a touchdown – in Sunday night's victory over the Houston Texans.Now, as the 49ers prepare to play the Arizona Cardinals Sunday at Candlestick Park, Brock again will be the team's third cornerback in passing situations.MORE: Aldon Smith Faces Felony Weapons, Misdemeanor DUI ChargesThis week, in fact, Brock was named the NFC Defensive Player of the Week for his play against the Texans, and 49ers head coach Jim Harbaugh said Brock will continue to get playing time."Really pleased with the way Tramaine's ascending," Harbaugh told the media this week. "When you play like that, you're going to continue to play. So, I think...