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 eeb1982c42258754f59c023bf2887bd8f77cf993..660e3e5aaf474313d61773ace2d170f1158489d2 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20435,6 +20435,14 @@ 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="FileBrowser.CloudImport.UserAction"
     enum="CloudImportUserAction">
   <owner>smckay@google.com</owner>
@@ -93174,6 +93182,14 @@ 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">Download succeeded.</int>
+  <int value="1" label="FAILED">Download failed.</int>
+  <int value="2" label="SKIPPED">
+    Download skipped because the URL was known to have failed previously.
+  </int>
+</enum>
+
 <enum name="FeatureObserver" type="int">
 <!-- Generated from third_party/WebKit/Source/core/frame/UseCounter.h -->