Add histograms for observing UMA throttling effect.

components/metrics/metrics_service.cc

Index: components/metrics/metrics_service.cc
diff --git a/components/metrics/metrics_service.cc b/components/metrics/metrics_service.cc
index 315229b11d1f0e73a6fcf399d4427d64aa47b748..24aa4f4a7ec4ed77b4d6ddfdc398bf0c270062be 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -882,13 +882,17 @@ void MetricsService::SendNextLog() {
 
   // Proceed to stage the log for upload if log size satisfies cellular log
   // upload constrains.
+  bool upload_canceled = false;
   if (client_->IsUMACellularUploadLogicEnabled() &&
       !data_use_tracker_->ShouldUploadLogOnCellular(
           log_manager_.staged_log_hash().size())) {
     scheduler_->UploadCancelled();
+    upload_canceled = true;
   } else {
     SendStagedLog();
   }
+  UMA_HISTOGRAM_BOOLEAN("UMA.LogUpload.Canceled.CellularConstraint",

[Alexei Svitkine (slow), 2016/04/11 15:09:08]

Let's not log this unless IsUMACellularUploadLogicEnabled() is true, so that
we're not logging this when it's not relevant.

[gayane -on leave until 09-2017, 2016/04/11 15:40:46]

Done.

+                        upload_canceled);
 }
 
 bool MetricsService::ProvidersHaveInitialStabilityMetrics() {