Support "shared" histograms between processes.

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 50545b3f68d2758e7307dac7edd23cb7d08a35e1..0c9bda693dc1810af9dacad9943f55751406e878 100644
--- a/components/metrics/metrics_service.cc
+++ b/components/metrics/metrics_service.cc
@@ -133,6 +133,7 @@
 #include "base/location.h"
 #include "base/metrics/histogram_base.h"
 #include "base/metrics/histogram_macros.h"
+#include "base/metrics/histogram_persistence.h"
 #include "base/metrics/histogram_samples.h"
 #include "base/metrics/sparse_histogram.h"
 #include "base/metrics/statistics_recorder.h"
@@ -740,6 +741,13 @@ void MetricsService::CloseCurrentLog() {
     OpenNewLog();  // Start trivial log to hold our histograms.
   }
 
+  // If a persistent allocator is in use, update its internal histograms (such
+  // as how much memory is being used) before reporting.
+  base::PersistentMemoryAllocator* allocator =
+      base::GetPersistentHistogramMemoryAllocator();
+  if (allocator)
+    allocator->UpdateStaticHistograms();
+
   // Put incremental data (histogram deltas, and realtime stats deltas) at the
   // end of all log transmissions (initial log handles this separately).
   // RecordIncrementalStabilityElements only exists on the derived
@@ -1100,12 +1108,14 @@ void MetricsService::RecordCurrentEnvironment(MetricsLog* log) {
 void MetricsService::RecordCurrentHistograms() {
   DCHECK(log_manager_.current_log());
   histogram_snapshot_manager_.PrepareDeltas(
+      base::StatisticsRecorder::begin(true), base::StatisticsRecorder::end(),
       base::Histogram::kNoFlags, base::Histogram::kUmaTargetedHistogramFlag);
 }
 
 void MetricsService::RecordCurrentStabilityHistograms() {
   DCHECK(log_manager_.current_log());
   histogram_snapshot_manager_.PrepareDeltas(
+      base::StatisticsRecorder::begin(true), base::StatisticsRecorder::end(),

[Alexei Svitkine (slow), 2016/01/18 19:25:36]

For all the calls to begin(), can you add a comment above explaining reasoning
for the true/false param, so it's obvious from the call site.

Particularly also the one in histogram_delta_serialization.cc.

[bcwhite, 2016/01/22 15:24:53]

Done.

       base::Histogram::kNoFlags, base::Histogram::kUmaStabilityHistogramFlag);
 }