[omnibox][ml][chrome://omnibox] Add chrome://omnibox/ml (6/6)

https://screenshot.googleplex.com/B36db39XGhcdxVz
https://screenshot.googleplex.com/3YZYX5mNgwM2Q76

This CL allows:
1. Comparing ML and traditional ranking side-by-side.
2. Seeing the ML signals and scores of all scored matches, instead of
   just the final matches.
3. Editing the signals of a match and seeing the resulting ML score.
4. Manually inputting ML signals and seeing the resulting ML score.

This required adding a new observer method on the
`AutocompleteController` called when a new ML batch is scored.

This only works with sync ML scoring. Supporting async ML scoring would
add complexity, and doesn't seem worth it.

(webUI that integrates with async cpp) and low value (it's a developer
tool).

Bug: 1405555
Change-Id: I964bd9d76373d9e54c500d42c139197652abc417
Low-Coverage-Reason: HARD_TO_TEST Adding tests for it has high cost
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4907777
Auto-Submit: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: manuk hovanesian <manukh@chromium.org>
Reviewed-by: Emily Stark <estark@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210572}