Move Singleton and related structs to namespace base

components/variations/net/variations_http_header_provider.h

Index: components/variations/net/variations_http_header_provider.h
diff --git a/components/variations/net/variations_http_header_provider.h b/components/variations/net/variations_http_header_provider.h
index 457aaabef6a9613fa0548716c32410bdb2e4a4aa..21f8659ea98e1ff3830bdd5ccee8754822c63150 100644
--- a/components/variations/net/variations_http_header_provider.h
+++ b/components/variations/net/variations_http_header_provider.h
@@ -26,7 +26,9 @@ class HttpRequestHeaders;
 
 class GURL;
 
+namespace base {
 template <typename T> struct DefaultSingletonTraits;
+}
 
 namespace variations {
 
@@ -61,7 +63,7 @@ class VariationsHttpHeaderProvider : public base::FieldTrialList::Observer,
   void ResetForTesting();
 
  private:
-  friend struct DefaultSingletonTraits<VariationsHttpHeaderProvider>;
+  friend struct base::DefaultSingletonTraits<VariationsHttpHeaderProvider>;
 
   FRIEND_TEST_ALL_PREFIXES(VariationsHttpHeaderProviderTest,
                            ShouldAppendHeaders);