Add DownloadStatus metric to FaviconHandler

components/favicon/core/favicon_handler.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_FAVICON_CORE_FAVICON_HANDLER_H_
 #define COMPONENTS_FAVICON_CORE_FAVICON_HANDLER_H_
 
 #include <stddef.h>
 
 #include <vector>
@@ skipping 57 matching lines @@
 //
 // When the renderer downloads favicons, it considers the entire list of
 // favicon candidates, if |download_largest_favicon_| is true, the largest
 // favicon will be used, otherwise the one that best matches the preferred size
 // is chosen (or the first one if there is no preferred  size). Once the
 // matching favicon has been determined, SetFavicon is called which updates
 // the page's favicon and notifies the database to save the favicon.
 
 class FaviconHandler {
  public:
+  // Outcome of a favicon download.
+  // Recorded as Favicons.DownloadOutcome and public for testing.
+  //
+  // These values must stay in sync with the FaviconDownloadStatus enum
+  // in histograms.xml.

[Ilya Sherman, 2017/04/12 23:22:14]

nit: Please also document that this enum should be treated as append-only, since
it backs an UMA histogram.

[fhorschig, 2017/04/13 15:14:05]

Done.

+  enum DownloadOutcome { SUCCEEDED = 0, FAILED = 1, SKIPPED = 2 };

[Ilya Sherman, 2017/04/12 23:22:14]

nit: Could this be an enum class?

[fhorschig, 2017/04/13 15:14:05]

Done.

+
   class Delegate {
    public:
     // Mimics WebContents::ImageDownloadCallback.
     typedef base::Callback<void(
         int id,
         int status_code,
         const GURL& image_url,
         const std::vector<SkBitmap>& bitmaps,
         const std::vector<gfx::Size>& original_bitmap_sizes)>
         ImageDownloadCallback;
@@ skipping 215 matching lines @@
   // UpdateFaviconCandidate()), the favicon service and the delegate are
   // notified.
   DownloadedFavicon best_favicon_;
 
   DISALLOW_COPY_AND_ASSIGN(FaviconHandler);
 };
 
 }  // namespace favicon
 
 #endif  // COMPONENTS_FAVICON_CORE_FAVICON_HANDLER_H_