Update the top domain list format

The current top domain list assumes that we can divide the list into
top-500 and non-top-500 buckets. We are planning to start using CrUX
data for the domain list which only has a top 1000 bucket, so this
assumption doesn't work anymore.

This CL slightly modifies the top domain list format and changes the
wording so that we can use an arbitrary number of domains in the very
top bucket.

Bug: 1151081
Change-Id: I3697f6649ad461e522eb39543101468f1e2e170d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4678750
Reviewed-by: Joe DeBlasio <jdeblasio@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1169445}