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 4b07c1f8c837bc8f48ae2af2b5dbfb3aedc5d863..56005dac1006de6efc7004fc4bd7becfc50f922b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -35558,6 +35558,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="OfflinePages.Background.OfflinerStatus"
+    enum="OfflinePagesBackgroundOfflinerStatus">

[fgorski, 2016/06/29 18:47:03]

suggestion: both enum and histogram name might be better with "Request"
mentioned someplace. 

[dougarnett, 2016/06/29 19:42:52]

Done.

+  <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>
@@ -83726,6 +83734,16 @@ 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="OfflinePagesBackgroundOfflinerStatus" type="int">
+  <int value="0" label="Unknown"/>
+  <int value="1" label="Loaded"/>
+  <int value="2" label="Saved"/>
+  <int value="3" label="Request canceled"/>
+  <int value="4" label="Load canceled"/>
+  <int value="5" label="Load 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"/>
@@ -97359,6 +97377,7 @@ To add a new entry, add it with any value and run test to compute valid value.
 <histogram_suffixes name="OfflinePagesNamespace" separator=".">
   <suffix name="bookmark" label="Offline bookmark cache"/>
   <suffix name="last_n" label="Offline recent pages"/>
+  <affected-histogram name="OfflinePages.Background.OfflinerStatus"/>
   <affected-histogram name="OfflinePages.DeletePage.AccessCount"/>
   <affected-histogram name="OfflinePages.DeletePage.LastOpenToCreated"/>
   <affected-histogram name="OfflinePages.DeletePage.PageSize"/>