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 d89cbc6bce6d96fc6e4d999bfcd5bf5cfceb00d3..881ea0b3d0035861b952cc5be9ce4e871b3fd3dd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -20560,6 +20560,14 @@ 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.LargeIconService.DownloadedSize" units="pixels">
   <owner>jkrcal@chromium.org</owner>
   <summary>
@@ -94784,6 +94792,17 @@ 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>
+  <int value="3" label="DOWNLOAD_OUTCOME_COUNT">
+    The amount of outcomes that can a download may have.
+  </int>
+</enum>
+
 <enum name="FaviconFetchResult" type="int">
   <int value="0" label="SUCCESS_CACHED">
     Success - favicon found in local cache