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..e1ed8d0093cc7588af8632c5ed492b9fc18ab419 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -1155,11 +1155,23 @@ 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);
+  for (auto it = base::StatisticsRecorder::begin(true),
+            end = base::StatisticsRecorder::end();
+       it != end; ++it) {
+    // kUmaStabilityHistogramFlag is actually a combination of two flags and
+    // so the result of the bitwise-and must be checked against the "flag"
+    // value or incorrect matches will occur.
+    if (((*it)->flags() & base::Histogram::kUmaStabilityHistogramFlag) ==
+        base::Histogram::kUmaStabilityHistogramFlag) {

[Alexei Svitkine (slow), 2016/05/10 19:55:42]

I would actually prefer we don't duplicate this logic that's already in
histogram_snapshot_manager_.PrepareDeltas().

Looks like the only difference is you don't want StartDeltas/FinishDeltas be
called before/after this and want to do it yourself. I suggest making another
version of PrepareDeltas() functions that doesn't call those.

For example, PrepareDeltasNoStartFinish().

Then, PrepareDeltas() can call that one surrounded by StartFinish() and this
file doesn't need to duplicate this logic.

[bcwhite, 2016/05/11 15:47:12]

Done.

+      histogram_snapshot_manager_.PrepareDelta(*it);
+    }
+  }
+  for (MetricsProvider* provider : metrics_providers_)
+    provider->RecordInitialHistogramSnapshots(&histogram_snapshot_manager_);
+  histogram_snapshot_manager_.FinishDeltas();
 }
 
 void MetricsService::LogCleanShutdown() {