Add DownloadStatus metric to FaviconHandler

components/favicon/core/favicon_handler.h

Index: components/favicon/core/favicon_handler.h
diff --git a/components/favicon/core/favicon_handler.h b/components/favicon/core/favicon_handler.h
index 61a159165f0cba26ca41a7c1bbabd4ac8567a8f6..8210298a393274da55f50c7b91dd2aabf5156a33 100644
--- a/components/favicon/core/favicon_handler.h
+++ b/components/favicon/core/favicon_handler.h
@@ -75,6 +75,13 @@ class FaviconService;
 
 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.