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

mojo/system/core.cc

Index: mojo/system/core.cc
diff --git a/mojo/system/core.cc b/mojo/system/core.cc
index 053c3fa2d875dae7ddb8c6b877ba19faa41fdb36..3679c6bbf512681d59b5563e978c732c96aa635e 100644
--- a/mojo/system/core.cc
+++ b/mojo/system/core.cc
@@ -200,10 +200,8 @@ MojoResult Core::WriteMessage(MojoHandle message_pipe_handle,
     return MOJO_RESULT_INVALID_ARGUMENT;
 
   // Easy case: not sending any handles.
-  if (num_handles == 0) {
-    return dispatcher->WriteMessage(bytes.GetPointerUnsafe(), num_bytes, NULL,
-                                    flags);
-  }
+  if (num_handles == 0)
+    return dispatcher->WriteMessage(bytes, num_bytes, NULL, flags);
 
   // We have to handle |handles| here, since we have to mark them busy in the
   // global handle table. We can't delegate this to the dispatcher, since the
@@ -237,8 +235,8 @@ MojoResult Core::WriteMessage(MojoHandle message_pipe_handle,
       return result;
   }
 
-  MojoResult rv = dispatcher->WriteMessage(bytes.GetPointerUnsafe(), num_bytes,
-                                           &transports, flags);
+  MojoResult rv = dispatcher->WriteMessage(bytes, num_bytes, &transports,
+                                           flags);
 
   // We need to release the dispatcher locks before we take the handle table
   // lock.