Mojo: Convert MemoryTest.Valid to the new user pointer handling (see r285350).

mojo/system/core.h

Index: mojo/system/core.h
diff --git a/mojo/system/core.h b/mojo/system/core.h
index 363b89732c3d3b15de427bb53b3314172e185d5a..963081c2f644dfbc1af1f60acbc4b30e5ce006ff 100644
--- a/mojo/system/core.h
+++ b/mojo/system/core.h
@@ -102,7 +102,7 @@ class MOJO_SYSTEM_IMPL_EXPORT Core {
                        uint64_t num_bytes,
                        UserPointer<void*> buffer,
                        MojoMapBufferFlags flags);
-  MojoResult UnmapBuffer(UserPointerValue<void> buffer);
+  MojoResult UnmapBuffer(UserPointer<void> buffer);
 
  private:
   friend bool internal::ShutdownCheckNoLeaks(Core*);