Add histograms for observing UMA throttling effect.

components/metrics/metrics_reporting_scheduler.h

Index: components/metrics/metrics_reporting_scheduler.h
diff --git a/components/metrics/metrics_reporting_scheduler.h b/components/metrics/metrics_reporting_scheduler.h
index f4e3dd11b3cf9baea7a87e5603b6fbd5d6bd98c4..c048a158e8c781c66900b3d60b509b97bdfa58d3 100644
--- a/components/metrics/metrics_reporting_scheduler.h
+++ b/components/metrics/metrics_reporting_scheduler.h
@@ -70,10 +70,6 @@ class MetricsReportingScheduler {
   // upload.
   base::TimeDelta upload_interval_;
 
-  // The tick count of the last time log upload has been finished and null if no
-  // upload has been done yet.
-  base::TimeTicks last_upload_finish_time_;
-
   // Indicates that the scheduler is running (i.e., that Start has been called
   // more recently than Stop).
   bool running_;