ScanningOption is an enum:

enum ScanningOption {
  SO_None = 0x0,
  SO_AllClasses = 0x1,
  SO_SystemClasses = 0x2,
  SO_Strings = 0x4,
  SO_AllCodeCache = 0x8,
  SO_ScavengeCodeCache = 0x10
};

but we combine the individual enum values into ScanningOption values that are not part of the enum definition.

E.g.:
   int so = SharedHeap::SO_AllClasses | SharedHeap::SO_Strings |
SharedHeap::SO_ScavengeCodeCache;

and later the cast "so", a set of values, to ScanningOption:

   gch->gen_process_strong_roots(_gen->level(),
                                 true, // Process younger gens, if any,
                                        // as strong roots.
                                 false, // [...]
----> SharedHeap::ScanningOption(so),
                                 &par_scan_state.to_space_root_closure(),
                                 &par_scan_state.older_gen_closure(),
                                 &klass_scan_closure);

We might want to clean this up so that we don't have to transition back and forth between ScanningOption (single value), int, and ScanningOption (combined values).