Adds UMA for PrerenderingOffliner request processing result status.

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 29f80d5f769686cac515e7857b1ed729a5d54c78..1cf97224a0532e1b54730e7d605d2e12c9054624 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35936,6 +35936,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="OfflinePages.Background.OfflinerRequestStatus"
+    enum="OfflinePagesBackgroundOfflinerRequestStatus">
+  <owner>dougarnett@chromium.org</owner>
+  <summary>
+    Status code of background offlining (loading and saving) requests.
+  </summary>
+</histogram>
+
 <histogram name="OfflinePages.BatchDelete.Count">
   <owner>jianli@chromium.org</owner>
   <summary>Number of offline pages that are deleted in a batch.</summary>
@@ -84471,6 +84479,16 @@ To add a new entry, add it with any value and run test to compute valid value.
       label="Account was not seeded before FireRefreshTokenAvailable"/>
 </enum>
 
+<enum name="OfflinePagesBackgroundOfflinerRequestStatus" type="int">

[Pete Williamson, 2016/07/11 20:12:08]

Should we also have a comment here pointing back to offliner.h?  (It's OK if the
answer is "no", I just want to make sure the question has been though about).

[dougarnett, 2016/07/11 21:06:55]

Done - added the "Generated from ..." style comment that seems to be the most
common in this file.

+  <int value="0" label="Unknown (not expected to be recorded)"/>
+  <int value="1" label="Loaded (not expected to be recorded)"/>
+  <int value="2" label="Saved"/>
+  <int value="3" label="RequestCoordinator canceled"/>
+  <int value="4" label="Prerendering canceled"/>
+  <int value="5" label="Prerendering failed"/>
+  <int value="6" label="Save failed"/>
+</enum>
+
 <enum name="OfflinePagesClearAllStatus" type="int">
   <int value="0" label="Success"/>
   <int value="1" label="Store reset failed"/>