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..59950a3b05bbe48e4bfbb640f58e1df9286577a2 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 +103061,55 @@ from previous Chrome versions.
   <int value="1" label="Offline"/>
 </enum>
 
+<enum name="OfflinePagesTabRestoreType" type="int">
+  <int value="0" label="While online">
+    Tab was successfully restored while the device was online. Can help assess
+    the potential benefit of allowing loading from offline pages even when the
+    device is online.
+  </int>
+  <int value="1" label="While online, can't be saved for offline usage">
+    Tab was successfully restored while the device was online but in a context
+    that wouldn't allow it to be saved as an offline page: not HTTP/HTTPS or in
+    an incognito tab.
+  </int>
+  <int value="2" label="While online, to an offline page">
+    Tab was successfully restored to a non-last_n offline page while the device
+    was online. This will happen when a tab is specifically loading a downloaded
+    offline page.
+  </int>
+  <int value="3" label="While online, to an offline page saved by last_n">
+    Tab was successfully restored to a last_n offline page while the device was
+    online. This will only happen if/when loading of last_n pages is allowed in
+    online situations.
+  </int>
+  <int value="4" label="While offline">
+    Tab was successfully restored while the device was offline. The page was
+    most probably loaded from cache without revalidation.
+  </int>
+  <int value="5" label="While offline, can't be saved for offline usage">
+    Tab was successfully restored while the device was offline but in a context
+    that wouldn't have allowed it to be saved as an offline page: not HTTP/HTTPS
+    or in an incognito tab. This includes tab restores to the NTP or other
+    chrome: pages.
+  </int>
+  <int value="6" label="While offline, to an offline page">
+    Tab was successfully restored to a non-last_n offline page while the device
+    was offline. This will happen when a tab had an associated offline page that
+    was not saved by last_n.
+  </int>
+  <int value="7" label="While offline, to an offline page saved by last_n">
+    Tab was successfully restored to a last_n offline page while the device was
+    offline.
+  </int>
+  <int value="8" label="Failed">
+    Tab failed being restored and an error page was presented. This represents
+    any kind of failure and not only dino pages.
+  </int>
+  <int value="9" label="Crashed">
+    The tab's renderer process crashed while it was being restored.
+  </int>
+</enum>
+
 <enum name="OfflineStatus" type="int">
   <obsolete>
     Deprecated 4/2015.