Convert WriteMessage...() to use the new user pointer handling (see r285350).

mojo/system/dispatcher.h

Index: mojo/system/dispatcher.h
diff --git a/mojo/system/dispatcher.h b/mojo/system/dispatcher.h
index dd5e2404e1673a80f63dec7ad7bd0117cd9ecd41..9162da0c72a7b694a3ff1c58af9590e77c2c7656 100644
--- a/mojo/system/dispatcher.h
+++ b/mojo/system/dispatcher.h
@@ -80,7 +80,7 @@ class MOJO_SYSTEM_IMPL_EXPORT Dispatcher :
   // written; not that |this| must not be in |transports|. On success, all the
   // dispatchers in |transports| must have been moved to a closed state; on
   // failure, they should remain in their original state.
-  MojoResult WriteMessage(const void* bytes,
+  MojoResult WriteMessage(UserPointer<const void> bytes,
                           uint32_t num_bytes,
                           std::vector<DispatcherTransport>* transports,
                           MojoWriteMessageFlags flags);
@@ -213,7 +213,7 @@ class MOJO_SYSTEM_IMPL_EXPORT Dispatcher :
   // See the descriptions of the methods without the "ImplNoLock" for more
   // information.
   virtual MojoResult WriteMessageImplNoLock(
-      const void* bytes,
+      UserPointer<const void> bytes,
       uint32_t num_bytes,
       std::vector<DispatcherTransport>* transports,
       MojoWriteMessageFlags flags);