[LargeIconService] Report size of favicon for fallback style to UMA

components/favicon/core/large_icon_service.cc

Index: components/favicon/core/large_icon_service.cc
diff --git a/components/favicon/core/large_icon_service.cc b/components/favicon/core/large_icon_service.cc
index c202c244ff784f4bb1634ee2b80da8a9e69e19b7..310d6f9143c9f8818912efeda0fee38664cef54c 100644
--- a/components/favicon/core/large_icon_service.cc
+++ b/components/favicon/core/large_icon_service.cc
@@ -117,10 +117,14 @@ void ProcessIconOnBackgroundThread(
     return;
 
   *fallback_icon_style = favicon_base::FallbackIconStyle();
+  int fallback_icon_size = 0;
   if (db_result.is_valid()) {
     favicon_base::SetDominantColorAsBackground(db_result.bitmap_data,
                                                fallback_icon_style);
+    fallback_icon_size = db_result.pixel_size.width();
   }
+  UMA_HISTOGRAM_COUNTS_100("Favicons.LargeIconService.FallbackSize",
+                           fallback_icon_size);

[Ilya Sherman, 2017/04/11 21:36:09]

Perhaps UMA_HISTOGRAM_SPARSE_SLOWLY, with some custom sanity-checks on the
logged value, would be more appropriate?

[jkrcal, 2017/04/12 05:40:51]

Done. 
No sanity checks are needed as this must be non-negative and relatively small
number (<100) by definition. If not, there are some bugs that we would like to
see :)

[Ilya Sherman, 2017/04/12 20:45:33]

Okay.  Could you please add a DCHECK at least?  =)

[jkrcal, 2017/04/13 07:16:05]

Done (I check only the lower limit, the upper limit depends on what the client
passes as |min_source_size| and I do not want to restrict something that the API
allows).

 }
 
 // Processes the bitmap data returned from the FaviconService as part of a