[amp-analytics] visiblePercentageMin in visibility spec does not default to 0

[lannka]

In the VisibilitySpec, if visiblePercentageMin not specified, visibility trigger fires as soon as the element laid out by runtime (not necessarily in viewport).

Is this a valid use case? Not sure if this metric (# of times an element get laid out) can be useful to pub, as this metric can suddenly change due to any change made in AMP runtime. For example, right now we lay out elements within 3 viewports. If we ever want to reduce this number, the metric will have a sudden drop.

On the other hand, it's confusing because I would think visiblePercentageMin defaults to 0. And I don't see it documented in the spec either.

Should we fix the behavior, or fix the doc?

[dvoytenko]

I agree. We should default it to 0.

[lannka]

If everyone agrees that we should default to 0, I will make it in the VisibilityV2 impl.

I'll let @avimehta to decide if we want to fix the current impl.