Measure time between secure time queries

components/network_time/network_time_tracker.cc

Index: components/network_time/network_time_tracker.cc
diff --git a/components/network_time/network_time_tracker.cc b/components/network_time/network_time_tracker.cc
index 89d2ad7f9d27e0912092d840b49ee7e29e410de1..785749267a9f22888ce8f3a9c0c5b35efdc56ff1 100644
--- a/components/network_time/network_time_tracker.cc
+++ b/components/network_time/network_time_tracker.cc
@@ -534,8 +534,19 @@ bool NetworkTimeTracker::UpdateTimeFromResponse() {
   base::TimeDelta resolution =
       base::TimeDelta::FromMilliseconds(1) +
       base::TimeDelta::FromSeconds(kTimeServerMaxSkewSeconds);
+
+  // Record histograms for the latency of the time query and the time delta
+  // between time fetches.
   base::TimeDelta latency = tick_clock_->NowTicks() - fetch_started_;
   UMA_HISTOGRAM_TIMES("NetworkTimeTracker.TimeQueryLatency", latency);
+  if (!last_fetched_time_.is_null()) {
+    UMA_HISTOGRAM_CUSTOM_TIMES("NetworkTimeTracker.TimeBetweenFetchedTimes",
+                               current_time - last_fetched_time_,
+                               base::TimeDelta::FromHours(1),
+                               base::TimeDelta::FromDays(7), 100);

[Ilya Sherman, 2016/12/13 22:02:07]

Hmm, do you really need 100 buckets, or would 50 suffice?  (Actually, 1 hour to
7 days is a very small range, so probably even fewer than 50 buckets would
suffice.)

[estark, 2016/12/13 22:09:36]

Changed to 50. (I take it there's a cost to having more buckets?)

[Ilya Sherman, 2016/12/14 01:34:43]

Yep, there's a cost on the client in terms of memory, and on the server in terms
of processing time.  It's not a big cost for any individual histogram; but we
have a lot of histograms in Chromium, and it adds up =)

+  }
+  last_fetched_time_ = current_time;
+
   UpdateNetworkTime(current_time, resolution, latency, tick_clock_->NowTicks());
   return true;
 }