Bug 1203314 - Make operator<< a method on unused_t. r=froydnj

This prevents operator overload resolution from failing when this file
is included in a file that uses Chromium IPC logging.

--HG--
extra : rebase_source : 661efe98060d405e87f828655b13c0c0a8a428e0

mfbt/unused.h

@@ -19,16 +19,13 @@ namespace mozilla {
 //
 struct unused_t
 {
+  template<typename T>
+  inline void
+  operator<<(const T& /*unused*/) const {}
 };
 
 extern MFBT_DATA const unused_t unused;
 
-template<typename T>
-inline void
-operator<<(const unused_t& /*unused*/, const T& /*unused*/)
-{
-}
-
 } // namespace mozilla
 
 #endif // __cplusplus