Add base::UnguessableToken << operator

base/unguessable_token.h

Index: base/unguessable_token.h
diff --git a/base/unguessable_token.h b/base/unguessable_token.h
index 64b1868886f2f0d06c613efa79a1f84ceb7b3c59..b1b3870723340d3dae94bee4f7e98b82efd1c3a3 100644
--- a/base/unguessable_token.h
+++ b/base/unguessable_token.h
@@ -7,6 +7,7 @@
 
 #include <stdint.h>
 #include <string.h>
+#include <iosfwd>
 #include <tuple>
 
 #include "base/base_export.h"
@@ -86,6 +87,9 @@ class BASE_EXPORT UnguessableToken {
   uint64_t low_ = 0;
 };
 
+BASE_EXPORT std::ostream& operator<<(std::ostream& out,
+                                     const UnguessableToken& token);
+
 // For use in std::unordered_map.
 struct UnguessableTokenHash {
   size_t operator()(const base::UnguessableToken& token) const {