The 49ers made some subtle alterations to their defense via free agency and the draft. And they are hopeful those changes help push them from being the top-13 group they were last season to one that cracks the top 10. For that to happen, though, they must stay healthy. That was not the case last year when Nick Bosa, Mykel Williams and Fred Warner missed significant time with injuries. The 49ers missed Mykel Williams on the field last season, when an injury limited him to nine games. Getty Images All three are on track to return, and if they stay on the field, it increases the chances the 49ers experience a big push defensively. Here is a look at where things stand with the 49ers’ defensive depth chart, after free agency and the draft : Defensive end Starters: Nick Bosa, Mykel Williams Reserves: Romello Height, Keion White, Sam Okuayinonu, Mikail Kamara, Andrew Farmer, Cameron Sample, William Bradley-King The skinny: It’s still a jolt realizing the 49ers managed just 20 sacks last year.…