Add a histogram for omnibox answer suggestion types.

components/omnibox/browser/search_suggestion_parser.cc

Index: components/omnibox/browser/search_suggestion_parser.cc
diff --git a/components/omnibox/browser/search_suggestion_parser.cc b/components/omnibox/browser/search_suggestion_parser.cc
index 32f7e5fee69d8139a8105a3d57358a6a2dc2d667..1ec248f0f1519d2cfd4bc3879dea878c89bb5f8b 100644
--- a/components/omnibox/browser/search_suggestion_parser.cc
+++ b/components/omnibox/browser/search_suggestion_parser.cc
@@ -524,6 +524,7 @@ bool SearchSuggestionParser::ParseSuggestResults(
             answer = SuggestionAnswer::ParseAnswer(answer_json);
             int answer_type = 0;
             if (answer && base::StringToInt(answer_type_str, &answer_type)) {
+              UMA_HISTOGRAM_SPARSE_SLOWLY("Omnibox.AnswerType", answer_type);

[Justin Donnelly, 2017/03/02 19:00:13]

Is this the right way to log this? These type IDs used to be sparse values like
2883 and 1372 so this is how I did it in the old Android-only histogram. Now
they're in a new, tight range (currently 1-10). But UMA_HISTOGRAM_ENUMERATION
doesn't seem right because the ID comes from the server so I can't guarantee
it's less than some man enum value. I guess I could define an enum in this file
and if the ID is greater than its max, log an error/0 case? Or is there some
other macro designed for this circumstance?

[Mark P, 2017/03/02 20:59:35]

Aren't there almost always zero or only one answer?
Hopefully sparse histograms are still the right solution.

[Ilya Sherman, 2017/03/02 21:17:32]

I recommend sparse histograms over enumerated histograms in most cases where the
histogram isn't being recorded in a tight loop.  So, yeah, I'd imagine that it's
a good choice here.

[Justin Donnelly, 2017/03/02 21:21:25]

Yes, there's always zero or one. When there's an answer, it includes an int type
and that type is in the range of 1-10 (defined as AnswerTriggererKey in
http://google3/logs/proto/suggest/suggest_extensions.proto).

Is there an implication of fact that there's only 0 or 1 answers I'm not
accounting for here?

[Mark P, 2017/03/02 23:28:48]

No "implication."  It just means that you're not calling
UMA_HISTOGRAM_SPARSE_SLOWLY() repeatedly in a tight loop.  You're calling it at
most once, so its possible slowness doesn't matter.

               answer_parsed_successfully = true;
 
               answer->set_type(answer_type);