How to set a nested strategy

[jasonwilliams]

Currently there's no straight forward way to set a nested strategy for ContainerTiming as the PerformanceObserver doesn't accept options. This may need to be a new field on PerformanceObserverInit

[jdapena]

The proposal with PerformanceObserverInit has two problems:

• It requires to change PerformanceObserverInit to add the new fields. This is shared with other timing events and would get a new field that is only usable for ContainerTiming.
• We cannot know the nesting policy when the events are happening. So, when painting happens, and the information is stored, we need to store all the information for the 3 nesting modes.

As a solution I am proposing (and implementing) a new approach that consist of defining a new attribute, containertiming-nesting for specifying the policy. This attribute gets the same values discussed before (ignore, transparent, shadowed). This way, when a paint needs to be processed, we already know the propagation rules, so we can immediately register the paint in all the roots that expect the value.