Move Singleton and related structs to namespace base

content/browser/histogram_synchronizer.cc

Index: content/browser/histogram_synchronizer.cc
diff --git a/content/browser/histogram_synchronizer.cc b/content/browser/histogram_synchronizer.cc
index 585179031e3b64c285ddc2a7c3fbc6405cb8d4a9..e0698b9154f9af119e8f8c7595ef333b32d32f7a 100644
--- a/content/browser/histogram_synchronizer.cc
+++ b/content/browser/histogram_synchronizer.cc
@@ -180,8 +180,9 @@ HistogramSynchronizer::~HistogramSynchronizer() {
 }
 
 HistogramSynchronizer* HistogramSynchronizer::GetInstance() {
-  return Singleton<HistogramSynchronizer,
-      LeakySingletonTraits<HistogramSynchronizer> >::get();
+  return base::Singleton<
+      HistogramSynchronizer,
+      base::LeakySingletonTraits<HistogramSynchronizer>>::get();
 }
 
 // static