Make the suppressed copy/assignment operators in MOJO_MOVE_ONLY_TYPE take const refs.

mojo/public/cpp/system/macros.h

Index: mojo/public/cpp/system/macros.h
diff --git a/mojo/public/cpp/system/macros.h b/mojo/public/cpp/system/macros.h
index 475ec9bc3d286cbd5b35045d8df13f5cc20deb77..36a3058c12157ce43d8ad9a75bba5dfdb4e7c034 100644
--- a/mojo/public/cpp/system/macros.h
+++ b/mojo/public/cpp/system/macros.h
@@ -35,8 +35,8 @@ char(&ArraySizeHelper(const T(&array)[N]))[N];
 // to tell that this type is move-only.
 #define MOJO_MOVE_ONLY_TYPE(type)                                              \
  private:                                                                      \
-  type(type&);                                                                 \
-  void operator=(type&);                                                       \
+  type(const type&);                                                           \
+  void operator=(const type&);                                                 \
                                                                                \
  public:                                                                       \
   type&& Pass() MOJO_WARN_UNUSED_RESULT { return static_cast<type&&>(*this); } \