[Cronet] Make HistogramManager public APIs thread-safe.

components/cronet/histogram_manager.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef COMPONENTS_CRONET_HISTOGRAM_MANAGER_H_
 #define COMPONENTS_CRONET_HISTOGRAM_MANAGER_H_
 
 #include <stdint.h>
 
 #include <string>
 #include <vector>
 
 #include "base/lazy_instance.h"
 #include "base/macros.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram_flattener.h"
 #include "base/metrics/histogram_snapshot_manager.h"
+#include "base/synchronization/lock.h"
 #include "components/metrics/proto/chrome_user_metrics_extension.pb.h"
 
 namespace cronet {
 
 // A HistogramManager instance is created by the app. It is the central
 // controller for the acquisition of log data, and recording deltas for
-// transmission to an external server.
+// transmission to an external server. Public APIs are all thread-safe.
 class HistogramManager : public base::HistogramFlattener {
  public:
   HistogramManager();
   ~HistogramManager() override;
 
   // Snapshot all histograms to record the delta into |uma_proto_| and then
   // returns the serialized protobuf representation of the record in |data|.
   // Returns true if it was successfully serialized.
   bool GetDeltas(std::vector<uint8_t>* data);
 
@@ skipping 10 matching lines @@
       base::HistogramBase::Inconsistency problem) override;
   void UniqueInconsistencyDetected(
       base::HistogramBase::Inconsistency problem) override;
   void InconsistencyDetectedInLoggedCount(int amount) override;
 
   base::HistogramSnapshotManager histogram_snapshot_manager_;
 
   // Stores the protocol buffer representation for this log.
   metrics::ChromeUserMetricsExtension uma_proto_;
 
+  // Should be acquired whenever GetDeltas() is executing to maintain
+  // thread-safety.
+  base::Lock get_deltas_lock_;
+
   DISALLOW_COPY_AND_ASSIGN(HistogramManager);
 };
 
 }  // namespace cronet
 
 #endif  // COMPONENTS_CRONET_HISTOGRAM_MANAGER_H_