Add DownloadStatus metric to FaviconHandler

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 11c010882138105792e9f76d6c650a7331cbdb53..59be1b5b063b489e3d1b27fcc74dc894a998018f 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20320,6 +20320,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Favicons.DownloadOutcome" enum="FaviconDownloadStatus">
+  <owner>fhorschig@chromium.org</owner>
+  <summary>
+    Records whether a download succeeded, failed or was skipped because it has
+    failed previously.
+  </summary>
+</histogram>
+
 <histogram name="Favicons.FaviconDownloadAttempts">
   <owner>fhorschig@chromium.org</owner>
   <summary>
@@ -92629,6 +92637,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </int>
 </enum>
 
+<enum name="FaviconDownloadStatus" type="int">
+  <int value="0" label="SUCCEEDED">A fitting icon was downloaded.</int>
+  <int value="1" label="FAILED">Downloading an icon failed.</int>
+  <int value="2" label="SKIPPED">
+    A download was skipped because the URL was known to have failed previously.
+  </int>

[jkrcal, 2017/04/10 12:29:38]

nit: Can you make the strings more coherent/analogous? E.g.
Download succeeded.
Download failed.
Download skipped because the URL was known to have failed previously.

[fhorschig, 2017/04/10 13:45:37]

Done. Thank you for the wording.

+</enum>
+
 <enum name="FeatureObserver" type="int">
 <!-- Generated from third_party/WebKit/Source/core/frame/UseCounter.h -->