[fetch-later] Update impl to "Reserve deferred-fetch quota" algo

The "subframe deferred fetch policy" implemented in [1] has been updated
to [2]: the spirit is the same, just that in the spec the
"DeferredFetchPolicy" has been explicitly mapped to a fixed quota. This
CL updates the implementation and adds util function
`ToReservedDeferredFetchQuota()` to preserve the mapping.

[1]: https://crrev.com/c/6055197
[2]: https://whatpr.org/fetch/1647.html#reserve-deferred-fetch-quota

Bug: 40276121
Change-Id: I3d30f94f8fca4060ad60fbda0e84e9d8952ffb1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6084052
Reviewed-by: Adam Rice <ricea@chromium.org>
Auto-Submit: Ming-Ying Chung <mych@chromium.org>
Commit-Queue: Ming-Ying Chung <mych@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1395743}