Update Session.TotalDuration description to mention known issue.

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 4e22c187711f934812506099cf16e385d94c65cb..ab11bf180093663c4cca5b2a5f9768625666d75b 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -59651,9 +59651,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <owner>chrisha@chromium.org</owner>
   <summary>
     The length of a session (launch/foregrounding to backgrounding) in
-    milliseconds. On desktop it also takes into account user interaction and
-    audio events. Starting from M55 the session length is recorded by
-    discounting for default inactivity timeout on desktop.
+    milliseconds.
+
+    Desktop: This also takes into account user interaction and audio events.
+    Starting from M55 the session length is recorded by discounting for default
+    inactivity timeout.
+
+    Android: There is a known issue where a JavaScript dialog can cause this
+    metric to record a session end prematurely. See: crbug.com/678276.
   </summary>
 </histogram>