Notify page load metrics when the app enters the background.

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 bfcd62d7744d22fe00b8b0be1dda12519e57b883..3b5c7a2db7d8704873a155871be0f503f1a53b96 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -37816,6 +37816,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.Internal.PageLoadCompleted.AfterAppBackground"
+    enum="Boolean">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Whether a page load completed after the app entered the background. We log a
+    'false' value for each PageLoadTracker that is active when the app enters
+    the background, and a 'true' value for each of those PageLoadTrackers that
+    successfully runs its destructor. This helps to understand how many trackers
+    are destroyed while the app is in the background before they are able to run
+    their destructor. This metric is only intended to help in understanding how
+    often app background leads to data loss, and will be deprecated in a
+    subsequent release. Please contact bmcquade@chromium.org before using this
+    metric.
+  </summary>
+</histogram>
+
 <histogram name="PageLoad.Internal.ProvisionalAbortChainSize.ForwardBack"
     units="length">
   <owner>csharrison@chromium.org</owner>