[Icon cacher] Add metrics for downloading favicons for NTP Tiles

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 3cb4e1df495072edb9f3a50c9caa48d1d4a48e68..5427e33976e0c2ca13429e1029dcb723dd3e2dff 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -44059,6 +44059,27 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.TileFaviconFetched.popular"
+    enum="NTPTileFaviconFetchResult">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Mobile only. The result of fetching a favicon for a tile on the New Tab
+    Page. This is recorded for each gray PopularSites tile on the New Tab Page
+    once per updating the set of tiles (notably upon each NTP impression).
+  </summary>
+</histogram>
+
+<histogram name="NewTabPage.TileFaviconFetched.server"
+    enum="NTPTileFaviconFetchResult">
+  <owner>jkrcal@chromium.org</owner>
+  <summary>
+    Mobile only. The result of fetching a favicon for a tile on the New Tab
+    Page. This is recorded for each gray MostVisited tile on the New Tab Page
+    once per lifetime of Chrome (there as an in-memory cache of failed
+    requests).
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.TileOfflineAvailable" enum="MostVisitedTileIndex">
   <owner>treib@chromium.org</owner>
   <summary>