Support saving browser metrics to disk and reading them during next run.

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 7e31ddb93156bf301ef333bd5cbe71bb7acc9691..a0a5658f2cd3652014dc07a0f8af9ac5aa263d82 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -1155,11 +1155,19 @@ void MetricsService::RecordCurrentHistograms() {
 
 void MetricsService::RecordCurrentStabilityHistograms() {
   DCHECK(log_manager_.current_log());
+  histogram_snapshot_manager_.StartDeltas();
   // "true" indicates that StatisticsRecorder should include histograms in
   // persistent storage.
-  histogram_snapshot_manager_.PrepareDeltas(
-      base::StatisticsRecorder::begin(true), base::StatisticsRecorder::end(),
-      base::Histogram::kNoFlags, base::Histogram::kUmaStabilityHistogramFlag);
+  auto end = base::StatisticsRecorder::end();

[Ilya Sherman, 2016/05/05 07:22:37]

nit: Please move this so that it's scoped to within the for loop.

[bcwhite, 2016/05/05 16:01:17]

Done.

+  for (auto it = base::StatisticsRecorder::begin(true); it != end; ++it) {
+    if (((*it)->flags() & base::Histogram::kUmaStabilityHistogramFlag) ==
+        base::Histogram::kUmaStabilityHistogramFlag) {
+      histogram_snapshot_manager_.PrepareDelta(*it);
+    }
+  }
+  for (MetricsProvider* provider : metrics_providers_)
+    provider->RecordInitialHistogramSnapshots(&histogram_snapshot_manager_);

[Ilya Sherman, 2016/05/05 07:22:37]

Providers might choose to include metrics that aren't stability metrics, in case
of a crash, right?  So, what's the purpose of checking the
kUmaStabilityHistogramFlag above?

[bcwhite, 2016/05/05 16:01:17]

You mean on line 1163?  That's always been there and means that only a small
subset of all the histograms known to the StatisticsRecorder (basically
everything created by this particular process) will get reported here.

In the case of the providers, it's up to them what metrics to include.  The
different calls of RecordInitialHsitogramSnapshots() vs
RecordHistogramSnapshots() allows providers to choose.

[Ilya Sherman, 2016/05/05 22:22:52]

Ah, I was misreading the code.  Thanks for explaining that in more detail :)

+  histogram_snapshot_manager_.FinishDeltas();
 }
 
 void MetricsService::LogCleanShutdown() {