Move Tuple to base namespace.

base/tuple.h

Index: base/tuple.h
diff --git a/base/tuple.h b/base/tuple.h
index 4628aa9417c395b472cfea0557af8b3da8b3fe3e..ef51d85fed9e7e5a5460a78595f245e3feeaef64 100644
--- a/base/tuple.h
+++ b/base/tuple.h
@@ -30,6 +30,8 @@
 
 #include "base/bind_helpers.h"
 
+namespace base {
+
 // Index sequences
 //
 // Minimal clone of the similarly-named C++14 functionality.
@@ -181,9 +183,9 @@ struct TupleLeaf {
 // Allows accessing an arbitrary tuple element by index.
 //
 // Example usage:
-//   Tuple<int, double> t2;
-//   get<0>(t2) = 42;
-//   get<1>(t2) = 3.14;
+//   base::Tuple<int, double> t2;
+//   base::get<0>(t2) = 42;
+//   base::get<1>(t2) = 3.14;
 
 template <size_t I, typename T>
 T& get(TupleLeaf<I, T>& leaf) {
@@ -329,4 +331,6 @@ inline void DispatchToMethod(ObjT* obj,
                        MakeIndexSequence<sizeof...(OutTs)>());
 }
 
+}  // namespace base
+
 #endif  // BASE_TUPLE_H_