Currently Serial GC does the ObjectCountAfterGC object iteration just after marking and class unloading.

As such it accesses unloaded classes during the object iteration, which is not nice. In JDK-8375314 this caused some issue/crashes, due to how class unloading is structured this does not affect Serial, but it is still not nice to access unloaded klasses (G1 and other collectors are not affected, they already skip objects with unloaded klasses during iteration).

Restructure the code similar to Parallel to avoid that.