Add ThreadChecker to HistogramSnapshotManager.

base/metrics/histogram_snapshot_manager.cc

Index: base/metrics/histogram_snapshot_manager.cc
diff --git a/base/metrics/histogram_snapshot_manager.cc b/base/metrics/histogram_snapshot_manager.cc
index a774ea6177c7da8ad8c95c9faded69c185502424..ae7f6b158fa5d32843dd2a9333b4ae831a448d6c 100644
--- a/base/metrics/histogram_snapshot_manager.cc
+++ b/base/metrics/histogram_snapshot_manager.cc
@@ -35,6 +35,7 @@ void HistogramSnapshotManager::PrepareFinalDelta(
 void HistogramSnapshotManager::PrepareSamples(
     const HistogramBase* histogram,
     std::unique_ptr<HistogramSamples> samples) {
+  DCHECK(thread_checker_.CalledOnValidThread());
   DCHECK(histogram_flattener_);
 
   // Get information known about this histogram. If it did not previously