Move Singleton and related structs to namespace base

base/trace_event/trace_event_etw_export_win.h

Index: base/trace_event/trace_event_etw_export_win.h
diff --git a/base/trace_event/trace_event_etw_export_win.h b/base/trace_event/trace_event_etw_export_win.h
index c1eafffa3a69ece14852aa1a18ff5ef4931b236d..7a1c02965d5d98d96216f93fcd923159ffd0fbe3 100644
--- a/base/trace_event/trace_event_etw_export_win.h
+++ b/base/trace_event/trace_event_etw_export_win.h
@@ -12,11 +12,11 @@
 #include "base/strings/string_piece.h"
 #include "base/trace_event/trace_event_impl.h"
 
-// Fwd.
+namespace base {
+
 template <typename Type>
 struct StaticMemorySingletonTraits;
 
-namespace base {
 namespace trace_event {
 
 class BASE_EXPORT TraceEventETWExport {
@@ -86,7 +86,7 @@ class BASE_EXPORT TraceEventETWExport {
   bool etw_export_enabled_;
 
   // Maps category names to their status (enabled/disabled).
-  std::map<base::StringPiece, bool> categories_status_;
+  std::map<StringPiece, bool> categories_status_;
 
   // Local copy of the ETW keyword.
   uint64 etw_match_any_keyword_;