Document FlattenableHelper design decision

    #pragma clang diagnostic ignored "-Wweak-vtables"

    #endif

    // FlattenableHelperInterface and FlattenableHelper avoid generating a vtable entry in objects

    // following Flattenable template/protocol.

    class FlattenableHelperInterface {

    protected:

        ~FlattenableHelperInterface() { }

    #pragma clang diagnostic pop

    #endif

    // Concrete implementation of FlattenableHelperInterface that delegates virtual calls to the

    // specified class T implementing the Flattenable protocol. It "virtualizes" a compile-time

    // protocol.

    template<typename T>

    class FlattenableHelper : public FlattenableHelperInterface {

        friend class Parcel;