Root margin parsing algorithm shouldn't allow non-pixel absolute lengths

[alijuma]

The "parse a root margin" algorithm says "If token is an absolute length dimension token, replace it with a an equivalent pixel length". But all implementations (Chrome, Edge, Firefox) throw exceptions on non-pixel absolute lengths (e.g., lengths in cm, mm, in, etc.). So the spec language needs to be updated.

[miketaylr]

Writing a test here to verify would be good, then we could get the spec text updated.

[miketaylr]

https://w3c.github.io/IntersectionObserver/#parse-a-root-margin percentage is allowed - but we should write tests to verify throwing for non px or non %.