Only positive frequency values should be accepted

[rakuco]

This came out of a discussion in https://chromium-review.googlesource.com/c/chromium/src/+/4506352 -- the spec currently has no requirements on whether SensorOptions["frequency"] should be > 0 even though a negative frequency value does not make much sense.

(It is important to note that, as stated in https://w3c.github.io/sensors/#initialize-a-sensor-object, specifying a frequency value in SensorOptions does not mean said value will be adopted by the implementation or used by the platform sensor)