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Currently, we persist to a pref when a user manually hides a suggestion
group. This works because all suggestion groups are default-shown.

In the near future, we plan to have some suggestion groups be
default-hidden. So now, we should persist if a user has manually shown
a suggestion group as well.

(See go/desktop-chrome-polaris-engdesign for details.)

This CL obsoletes the "omnibox.hiddenGroupIds" list pref and instead
creates a new "omnibox.suggestionGroupVisibility" dictionary pref,
which has the group ID as the key, and stores two possible states:
 - HIDDEN (1)
 - SHOWN (2)

If the user has never set the preference, we plan to eventually return
a third value:
 - DEFAULT (0)

In that case, the callsite will have to decide on the proper default
visibility, which will be either a client-side flag or a
server-provided hint per-group.

One more thing:

I've purposely not provided a migration path from the old pref to the
new pref. I did this for simplicity sake. I've looked at the UMA, and
there are only ~25 count usages of the collapse / expand feature each
on Stable in the last 30 days.

Across all Channels, there's ~550 each over the last 30 days. I think
a significant fraction of that 550 is Chromium developers.

In other words, I think simplicity trumps migrating the prefs of the
scant few users who've set this pref so far.

Bug: 1106096
Change-Id: I8f0bf559f897474c18b9bf90cc33cb8b040cb0fa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303862
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790554}