Move Singleton and related structs to namespace base

content/common/sandbox_linux/sandbox_linux.h

Index: content/common/sandbox_linux/sandbox_linux.h
diff --git a/content/common/sandbox_linux/sandbox_linux.h b/content/common/sandbox_linux/sandbox_linux.h
index 01a0bd9a3546552fddbe2fea9c8c468d9bb3ab9d..2bbc67c255746d27b462cd3079d6fd284b6346c5 100644
--- a/content/common/sandbox_linux/sandbox_linux.h
+++ b/content/common/sandbox_linux/sandbox_linux.h
@@ -20,8 +20,9 @@
 #define ANY_OF_AMTLU_SANITIZER 1
 #endif
 
-template <typename T> struct DefaultSingletonTraits;
 namespace base {
+template <typename T>
+struct DefaultSingletonTraits;
 class Thread;
 }
 namespace sandbox { class SetuidSandboxClient; }
@@ -133,7 +134,7 @@ class LinuxSandbox {
 #endif
 
  private:
-  friend struct DefaultSingletonTraits<LinuxSandbox>;
+  friend struct base::DefaultSingletonTraits<LinuxSandbox>;
 
   LinuxSandbox();
   ~LinuxSandbox();