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..dfe2f86492a577b40aaaf8e2b61dbb196143c26c 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -886,6 +886,7 @@ void MetricsService::SendNextLog() {
       !data_use_tracker_->ShouldUploadLogOnCellular(
           log_manager_.staged_log_hash().size())) {
     scheduler_->UploadCancelled();
+    UMA_HISTOGRAM_COUNTS("UMA.LogUpload.Canceled.CellularConstraint", 1);

[Alexei Svitkine (slow), 2016/04/08 15:08:32]

UMA_HISTOGRAM_COUNTS() allocated 50 buckets. You can use
UMA_HISTOGRAM_BOOLEAN(..., true); to log the same data more efficiently.

I suggest logging both the true and false cases, so we can get a ratio.

[gayane -on leave until 09-2017, 2016/04/08 17:19:25]

I have added the false case here, but it wouldn't mean that the log will
certainly get uploaded.

   } else {
     SendStagedLog();
   }