[omnibox][ml] Cull per ML ranking instead of traditional ranking

go/autocomplete-controller-refactor has some details on the refactor
plan.

This is part 7 of the refactor.

Before this CL, autocomplete passes with transferred matches looked
like:

1. Sort, dedupe, cull, etc the new matches
2. Append the old matches to the new matches
3. ML score the merged matches
4. Sort, dedupe, cull, etc the merged matches

This was problematic, because step 1 would cull matches based on
traditional scores instead of the ML scores. So it could lead to culling
a match that's scored higher by the ML model than another match that
wasn't culled.

This didn't affect the autocomplete passes without transferred matches,
since those only do steps 3 & 4.

This CL moves ML scoring (step 3) before the initial culling (step 1):

3. ML score the new matches
2. Sort, dedupe, cull, etc the new matches
3. Append the old matches to the new matches
4. Sort, dedupe, cull, etc the merged matches

This also helps avoid overwriting the score demotions
`TransferOldMatches()` does.

This is not feature guarded, because the impact on ranking is probably
small.

Bug: 1405555
Change-Id: I831a67f5ee7004d4042029a67d7d946ffe99c172
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5154360
Reviewed-by: Angela Yoeurng <yoangela@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1242481}