When network time is unavailable, record the reason in UMA

components/network_time/network_time_tracker.h

Index: components/network_time/network_time_tracker.h
diff --git a/components/network_time/network_time_tracker.h b/components/network_time/network_time_tracker.h
index 07869e3f271b13e240bba1295fbb421b08dc8e62..730ece825844a2314d7a840ce19002a226aaf803 100644
--- a/components/network_time/network_time_tracker.h
+++ b/components/network_time/network_time_tracker.h
@@ -48,6 +48,20 @@ const int64_t kTicksResolutionMs = 1;  // Assume 1ms for non-windows platforms.
 // for a corresponding local time. This class is not thread safe.
 class NetworkTimeTracker : public net::URLFetcherDelegate {
  public:
+  // Describes the result of a GetNetworkTime() call, describing whether
+  // network time was available and if not, why not.
+  enum NetworkTimeResult {
+    // Network time is available.
+    NETWORK_TIME_AVAILABLE,
+    // A time has been retrieved from the network in the past, but
+    // network time is no longer available because the tracker fell out
+    // of sync due to, for example, a suspend/resume.
+    NETWORK_TIME_SYNC_LOST,

[mab, 2016/08/16 19:04:52]

I was thinking this morning that if we've once obtained sync, and if we trust
the TickClock, we at least know a lower bound on the time.  I don't know if this
is useful.  :-)

[estark, 2016/08/18 12:15:11]

Acknowledged. Might be useful; I was also thinking that if it turns out that we
just can't keep sync well, then maybe we should revisit the strategy of querying
on-demand when we hit ERR_CERT_DATE_INVALID...

+    // Network time is unavailable because the tracker has not yet
+    // retrieved a time from the network.
+    NETWORK_TIME_NO_SYNC,

[mab, 2016/08/16 19:04:52]

There's an implementation detail here that you may or may not want to expose. 
Either the TickClock is ahead of the Clock, or the Clock is ahead of the
TickClock.  Might it be useful to know which?

[estark, 2016/08/18 12:15:11]

It occurred to me that knowing the amount of divergence might be useful (maybe
kClockDivergenceSeconds is a little too small and we could maintain "sync" a lot
more often if we relax the definition of "synced"). So I added another histogram
to record the difference of the two clocks, which will also reveal which was
ahead of the other.

+  };
+
   static void RegisterPrefs(PrefRegistrySimple* registry);
 
   // Constructor.  Arguments may be stubbed out for tests.  |getter|, if not
@@ -59,9 +73,13 @@ class NetworkTimeTracker : public net::URLFetcherDelegate {
                      scoped_refptr<net::URLRequestContextGetter> getter);
   ~NetworkTimeTracker() override;
 
-  // Sets |network_time| to an estimate of the true time.  Returns true if time
-  // is available, and false otherwise.  If |uncertainty| is non-NULL, it will
-  // be set to an estimate of the error range.
+  // Sets |network_time| to an estimate of the true time.  Returns
+  // NETWORK_TIME_AVAILABLE if time is available. If |uncertainty| is
+  // non-NULL, it will be set to an estimate of the error range.
+  //
+  // If network is unavailable, this method returns
+  // NETWORK_TIME_SYNC_LOST or NETWORK_TIME_NO_SYNC to indicate the
+  // reason.
   //
   // Network time may be available on startup if deserialized from a pref.
   // Failing that, a call to |UpdateNetworkTime| is required to make time
@@ -69,8 +87,8 @@ class NetworkTimeTracker : public net::URLFetcherDelegate {
   // become unavailable if |NetworkTimeTracker| has reason to believe it is no
   // longer accurate.  Consumers should even be prepared to handle the case
   // where calls to |GetNetworkTime| never once succeeds.
-  bool GetNetworkTime(base::Time* network_time,
-                      base::TimeDelta* uncertainty) const;
+  NetworkTimeResult GetNetworkTime(base::Time* network_time,
+                                   base::TimeDelta* uncertainty) const;

[mab, 2016/08/16 19:04:52]

Most callers don't care about the reason for failure.  You could do this if you
wanted to avoid changing callers. But there are so few callers maybe it isn't
worth it.

bool GetNetworkTime(base::Time* network_time, base::TimeDelta* uncertainty,
NetworkTimeResult* status = nullptr)

[estark, 2016/08/18 12:15:11]

Acknowledged. Decided to leave as-is because I already changed the small number
of callers, plus I think default arguments might be discouraged or forbidden in
Chromium.

 
   // Calculates corresponding time ticks according to the given parameters.
   // The provided |network_time| is precise at the given |resolution| and