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

If it's good for Chromium (https://codereview.chromium.org/1061073004),
it's good for us, right?

R=kulakowski@chromium.org

Review URL: https://codereview.chromium.org/1352683002 .

mojo/public/cpp/system/macros.h

diff --git a/mojo/public/cpp/system/macros.h b/mojo/public/cpp/system/macros.h
index 475ec9b..36a3058 100644
--- a/mojo/public/cpp/system/macros.h
+++ b/mojo/public/cpp/system/macros.h
@@ -35,8 +35,8 @@
 // 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); } \