Extract common code from FlagRegisterer to reduce size.
Extract common code from FlagRegisterer to reduce size.

fuchsia-mirror/third_party-gflags,schuhschuh/gflags,n054/gflags,n054/gflags,fuchsia-mirror/third_party-gflags,n054/gflags,fuchsia-mirror/third_party-gflags,gflags/gflags,n054/gflags,gflags/gflags,gflags/gflags,schuhschuh/gflags,fuchsia-mirror/third_party-gflags,schuhschuh/gflags,gflags/gflags,schuhschuh/gflags