Declares that the current module will not define any more key flags.
Normally, the module that calls the DEFINE_xxx functions claims the flag to be its key flag. This is undesirable for modules that define additional DEFINE_yyy functions with its own flag parsers and serializers, since that module will accidentally claim flags defined by DEFINE_yyy as its key flags. After calling this function, the module disclaims flag definitions thereafter, so the key flags will be correctly attributed to the caller of DEFINE_yyy.
After calling this function, the module will not be able to define any more flags. This function will affect all FlagValues objects.