Make Version of Session.TotalDuration with Higher Max (for desktop)

This is important for desktop, as >1% of events emitted are hitting the
max.  By my estimates from looking at the distribution, it's probably
safe to change the max to more like 6-10 hours to have fewer than 1% of
emits on desktop in the overflow bucket.  I decided to use one day
as the max simply because it will make it easier to identify possible
issues in the future with this metric.  (E.g., we should not see
two emits to the overflow bucket on a single day, otherwise something
is wrong.)

While at it, revise Session.TotalDuration description to explain how
it changed in M68.

BUG=856467

Change-Id: If0fcead52a15af24899618448b9c72bb87fb3e2b
Reviewed-on: https://chromium-review.googlesource.com/1147210Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#577611}

chrome/browser/metrics/desktop_session_duration/desktop_session_duration_tracker.cc

@@ -164,6 +164,10 @@ void DesktopSessionDurationTracker::EndSession(
   // Note: This metric is recorded separately for Android in
   // UmaSessionStats::UmaEndSession.
   UMA_HISTOGRAM_LONG_TIMES("Session.TotalDuration", delta);
+
+  UMA_HISTOGRAM_CUSTOM_TIMES("Session.TotalDurationMax1Day", delta,
+                             base::TimeDelta::FromMilliseconds(1),
+                             base::TimeDelta::FromHours(24), 50);
 }
 
 void DesktopSessionDurationTracker::InitInactivityTimeout() {

tools/metrics/histograms/histograms.xml

@@ -91281,7 +91281,8 @@ uploading your change for review.
 
     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.
+    inactivity timeout.  Also, until M69, there was a bug that caused
+    double-counting of some sessions.
 
     Android: There is a known issue where JavaScript dialogs and other things
     that take focus away from the Chrome activity can cause this metric to
@@ -91445,6 +91446,23 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Session.TotalDurationMax1Day" units="ms">
+  <owner>asvitkine@chromium.org</owner>
+  <owner>chrisha@chromium.org</owner>
+  <summary>
+    The length of a session (launch/foregrounding to backgrounding) in
+    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.  Also, until M69, there was a bug that caused
+    double-counting of some sessions.
+
+    Android: This histogram isn't logged on Android.  See instead
+    Session.TotalDuration.
+  </summary>
+</histogram>
+
 <histogram name="SessionCrashed.Bubble" enum="SessionCrashedBubbleUserAction">
   <owner>yiyaoliu@chromium.org</owner>
   <summary>How did the user interact with the SessionCrashed Bubble?</summary>