[resize-observer] In shipping browsers, observe() doesn't call unobserve()

Per the current spec, observe(target, options) is suppose to call unobserve(target) if target is already in the observationTargets.

From my understanding, this should clear the lastReportedSizes array for the target. Hence, even if the target hasn't changed size, re-calling observe with the target should always fire an observation.

In practice, all major web browsers do not appear to implement this. Instead, to get the observation to fire, I must manually call unobserve(target) first and then call observe(target) again.

In this example, adding or removing a column should always trigger an observation. However, it only triggers an observation if I explicitly call unobserve() first.

Start with the ResizeObserver observe() and unobserve() algorithms in the current specification, then reproduce the behavior using the linked JSFiddle example. Compare the specification's expected handling of observationTargets and lastReportedSizes with shipping-browser behavior; done means the discrepancy and required specification change are resolved or clearly documented.