Add ThreadChecker 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 de4a2e195a1a725a789f0cd346ae84f023681881..b2cd85cfd8126954fb8408ee66c090919ec88a81 100644
--- a/base/metrics/histogram_snapshot_manager.h
+++ b/base/metrics/histogram_snapshot_manager.h
@@ -14,6 +14,7 @@
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/metrics/histogram_base.h"
+#include "base/threading/thread_checker.h"
 
 namespace base {
 
@@ -88,6 +89,8 @@ class BASE_EXPORT HistogramSnapshotManager {
   // deltas.
   HistogramFlattener* histogram_flattener_;  // Weak.
 
+  ThreadChecker thread_checker_;
+
   DISALLOW_COPY_AND_ASSIGN(HistogramSnapshotManager);
 };