Add stream logging support for base::Optional

base/optional.h

Index: base/optional.h
diff --git a/base/optional.h b/base/optional.h
index b468964ae339fbead38f769dd315ff39a008d054..2e322dee6d785e6caca8740abb4f2bc44f468176 100644
--- a/base/optional.h
+++ b/base/optional.h
@@ -5,6 +5,7 @@
 #ifndef BASE_OPTIONAL_H_
 #define BASE_OPTIONAL_H_
 
+#include <ostream>
 #include <type_traits>
 
 #include "base/logging.h"
@@ -436,6 +437,16 @@ constexpr Optional<typename std::decay<T>::type> make_optional(T&& value) {
   return Optional<typename std::decay<T>::type>(std::forward<T>(value));
 }
 
+// For logging use only.
+template <class T>
+std::ostream& operator<<(std::ostream& os, const Optional<T>& opt) {
+  if (opt)
+    os << opt.value();
+  else
+    os << "(unset)";
+  return os;
+}
+
 template <class T>
 void swap(Optional<T>& lhs, Optional<T>& rhs) {
   lhs.swap(rhs);