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We want to differentiate between first show attempt/result and
subsequent re-show attempts/results in UMA histograms for proactive
suggestions so that we can measure engagement of users the first time
they are presented w/ content in comparison to re-presentations of the
same content.

To accomplish this we add:
- Assistant.ProactiveSuggestions.FirstShowAttempt
- Assistant.ProactiveSuggestions.FirstShowAttempt.<Prefix>.ByCategory
- Assistant.ProactiveSuggestions.FirstShowResult
- Assistant.ProactiveSuggestions.FirstShowResult.<Prefix>.ByCategory
- Assistant.ProactiveSuggestions.ReshowAttempt
- Assistant.ProactiveSuggestions.ReshowAttempt.<Prefix>.ByCategory
- Assistant.ProactiveSuggestions.ReshowResult
- Assistant.ProactiveSuggestions.ReshowResult.<Prefix>.ByCategory

We mark the following pre-existing histograms as obsolete:
- Assistant.ProactiveSuggestions.ShowAttempt
- Assistant.ProactiveSuggestions.ShowAttempt.<Suffix>
- Assistant.ProactiveSuggestions.ShowResult
- Assistant.ProactiveSuggestions.ShowResult.<Suffix>

Bug: b:144875278
Change-Id: I8af637b43bb485d8ed1a7356311dd25a9aabb7e3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1951549
Commit-Queue: David Black <dmblack@google.com>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#722688}