[Offline pages on the NTP] Add UMA for offline-available 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 d1f6c589c98f2ed52d062650af328089eaa1f7c7..d1568a758a272e9f1650a328e24841141e05b679 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -31128,6 +31128,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="NewTabPage.TileOfflineAvailable">

[Alexei Svitkine (slow), 2016/02/22 20:47:27]

Nit: Since this is more like an enum histogram - since you're logging the tab
index (and doing thinkg like percentiles / means / etc - doesn't make sense on
it) can you add an <enum> entry and specify it here via enum=""? This will let
our dashboards provide better views for it.

[Marc Treib, 2016/02/23 10:11:40]

With enum labels like "Tile 1", "Tile 2", or probably just "0", "1"?
I guess it makes sense to get rid of the non-helpful means etc. But then we
should do it for the other similar histograms (e.g.
NewTagPage.SuggestionsImpression) as well, to be consistent. I'd like to do all
of them in a followup CL, if that's okay with you?

+  <owner>treib@chromium.org</owner>
+  <summary>
+    The number of times a tile was available offline, per tile index - compare
+    to the NewTabPage.SuggestionsImpression.* histograms. This is recorded when
+    the NTP finishes loading. Only measured on Android.
+  </summary>
+</histogram>
+
 <histogram name="NewTabPage.TileType" enum="MostVisitedTileType">
   <owner>newt@chromium.org</owner>
   <summary>