Add concurrency check to HistogramSnapshotManager.

base/metrics/histogram_snapshot_manager.h

Index: base/metrics/histogram_snapshot_manager.h
diff --git a/base/metrics/histogram_snapshot_manager.h b/base/metrics/histogram_snapshot_manager.h
index fe26faad1138c6dbaa57f40b9d23c56eeb3ecb94..51bf92c7b55aa1bc81cf8b6b79a300640b3cbcb9 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -7,6 +7,7 @@
 
 #include <stdint.h>
 
+#include <atomic>
 #include <map>
 #include <string>
 #include <vector>
@@ -14,7 +15,6 @@
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
-#include "base/threading/thread_checker.h"
 
 namespace base {
 
@@ -76,15 +76,19 @@ class BASE_EXPORT HistogramSnapshotManager {
   void PrepareSamples(const HistogramBase* histogram,
                       std::unique_ptr<HistogramSamples> samples);
 
+  // |histogram_flattener_| handles the logistics of recording the histogram
+  // deltas.
+  HistogramFlattener* const histogram_flattener_;  // Weak.
+
   // For histograms, track what has been previously seen, indexed
   // by the hash of the histogram name.
   std::map<uint64_t, SampleInfo> known_histograms_;
 
-  // |histogram_flattener_| handles the logistics of recording the histogram
-  // deltas.
-  HistogramFlattener* histogram_flattener_;  // Weak.
-
-  ThreadChecker thread_checker_;
+  // A flag indicating if a thread is currently doing an operation. This is
+  // used to check against concurrent access which is not supported. A Thread-
+  // Checker is not sufficient because it may be guarded by at outside lock
+  // (as is the case with cronet).
+  std::atomic<bool> is_active_;
 
   DISALLOW_COPY_AND_ASSIGN(HistogramSnapshotManager);
 };