Move Singleton and related structs to namespace base

content/browser/mach_broker_mac.mm

Index: content/browser/mach_broker_mac.mm
diff --git a/content/browser/mach_broker_mac.mm b/content/browser/mach_broker_mac.mm
index 3efec6b94d6b589074a77581b43d87917b45eb6d..b41c6032e53a76146a92a3a79d7870a366819e9c 100644
--- a/content/browser/mach_broker_mac.mm
+++ b/content/browser/mach_broker_mac.mm
@@ -78,7 +78,8 @@ bool MachBroker::ChildSendTaskPortToParent() {
 }
 
 MachBroker* MachBroker::GetInstance() {
-  return Singleton<MachBroker, LeakySingletonTraits<MachBroker>>::get();
+  return base::Singleton<MachBroker,
+                         base::LeakySingletonTraits<MachBroker>>::get();
 }
 
 base::Lock& MachBroker::GetLock() {