Adds metric to help assess last_n impact on tab restores.

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 35bf3ee00b5c96f35104ccaf8a92476c6e6b2fea..220ed24ac4a84ff45bed3457d5e6d533312523d0 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -43493,6 +43493,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="OfflinePages.TabRestore" enum="OfflinePagesTabRestoreType">
+  <owner>carlosk@chromium.org</owner>
+  <summary>
+    Tracks the reload of contents of an existing tab that is being restored
+    either because that tab is being presented for the first time after Chrome
+    was restarted or because its contents were discarded. All buckets are
+    mutually exclusive (adding up all reported values amounts to the total
+    number of tracked tab restores). This metric is focused on last_n offline
+    pages and is exclusive to Chrome on Android.
+  </summary>
+</histogram>
+
 <histogram name="OfflinePages.TotalPageSize" units="MB">
   <owner>fgorski@chromium.org</owner>
   <owner>jianli@chromium.org</owner>
@@ -103049,6 +103061,20 @@ from previous Chrome versions.
   <int value="1" label="Offline"/>
 </enum>
 
+<enum name="OfflinePagesTabRestoreType" type="int">
+  <int value="0" label="WhileOnline"/>

[fgorski, 2017/03/17 03:42:51]

This can have spaces and sentence case.

[carlosk, 2017/03/17 20:37:22]

Done. Indeed much better! I also learned about and added descriptions to each
item that show up as hover text in UMA.

+  <int value="1" label="WhileOnlineCouldBeSavedOffline"/>
+  <int value="2" label="WhileOnlineToOfflinePage"/>
+  <int value="3" label="WhileOnlineToOfflinePageFromLastN"/>
+  <int value="4" label="WhileOffline"/>
+  <int value="5" label="WhileOfflineCouldBeSavedOffline"/>
+  <int value="6" label="WhileOfflineToOfflinePage"/>
+  <int value="7" label="WhileOfflineToOfflinePageFromLastN"/>
+  <int value="8" label="Failed"/>
+  <int value="9" label="FailedToDinoPage"/>
+  <int value="10" label="Crashed"/>
+</enum>
+
 <enum name="OfflineStatus" type="int">
   <obsolete>
     Deprecated 4/2015.