Consider not favoring new sources when there is an existing attributed source

[johnivdel]

In step 2 of this algorithm, https://wicg.github.io/attribution-reporting-api/#process-an-attribution-source, the browser drops sources which have already been attributed when there is a new matching source.

This logic predates the priority mechanism, and this may not be expected in all instances. For example, consider a flow like:

source A (click) , time = 0 day, priority = 10 conversion, page visit (generates report 1) source B, time = 0, priority = 2 conversion, purchase

In this example, the purchase conversion would be attributed to source B, despite it being lower priority and source A still having the ability to create reports.