Low-cost Up-Down Servo solution
The smallest, cheapest, most reliable solution will use a small, 6-pin microcontroller: VCC, GND, input, output, with two pins left to spare.

If you're not after necessarily smallest nor the cheapest, but are after low-tech, then a double 555 (say LMC556/TLC556) would do: one half would act as a toggle F-F, another one as a PWM generator, using the input from the first half to change the pulse width. This has a bunch of passive components, so it will cost more than a little 6 pin micro that only needs a capacitor to run.

An implementation using a cheap double (or quadruple) op-amp would work as well - again, at the reliability cost of passive parts.