tl;dr IntersectionObserver will now support the "thresholds" attribute, as dictated by the
spec.
The IntersectionObserver implementation which shipped in M51 is missing the "thresholds" attribute from the
specification. This was a conscious omission, as at the time blink IDL did not yet support FrozenArray types.
Now that FrozenArray is fully supported by blink IDL,
this change will add the missing "thresholds" attribute. With a bit of luck, this change will ship in M52. There is currently no plan to back port it to M51.