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 daf27f8b2df08bb09fb1f688ce9baab0c31ec5f8..2b20ca9059befbe1927daabda55d9f5a4d14151a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -37827,6 +37827,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.Internal.PageLoadCompleted.AfterAppBackground"
+    enum="BooleanStartedCompleted">
+  <owner>bmcquade@chromium.org</owner>
+  <summary>
+    Whether a page load completed after the app entered the background. We log a
+    'Started' value for each PageLoadTracker that is active when the app enters
+    the background, and a 'Completed' 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>
@@ -68295,6 +68311,11 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="1" label="Stale"/>
 </enum>
 
+<enum name="BooleanStartedCompleted" type="int">
+  <int value="0" label="Started"/>
+  <int value="1" label="Completed"/>
+</enum>
+
 <enum name="BooleanStreamed" type="int">
   <int value="0" label="Not streamed"/>
   <int value="1" label="Streamed"/>