Use ThreadChecker in HistogramDeltaSerialization class.

base/metrics/histogram_delta_serialization.h

Index: base/metrics/histogram_delta_serialization.h
diff --git a/base/metrics/histogram_delta_serialization.h b/base/metrics/histogram_delta_serialization.h
index a379914c0db48c23be5c910330c026a507431145..0bc4a5905fb8470cbd1c9884da006b6cdf57209e 100644
--- a/base/metrics/histogram_delta_serialization.h
+++ b/base/metrics/histogram_delta_serialization.h
@@ -13,6 +13,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram_flattener.h"
 #include "base/metrics/histogram_snapshot_manager.h"
+#include "base/threading/thread_checker.h"
 
 namespace base {
 
@@ -45,6 +46,8 @@ class BASE_EXPORT HistogramDeltaSerialization : public HistogramFlattener {
       HistogramBase::Inconsistency problem) override;
   void InconsistencyDetectedInLoggedCount(int amount) override;
 
+  ThreadChecker thread_checker_;
+
   // Calculates deltas in histogram counters.
   HistogramSnapshotManager histogram_snapshot_manager_;