Mojo: Convert MemoryTest.Valid to 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 9b91632b946f3e01768c5d793568b4e1df5fbe00..0f006eb189393ca4f765f54d207d7327bf71ee3f 100644
--- a/mojo/system/core.cc
+++ b/mojo/system/core.cc
@@ -500,9 +500,9 @@ MojoResult Core::MapBuffer(MojoHandle buffer_handle,
   return MOJO_RESULT_OK;
 }
 
-MojoResult Core::UnmapBuffer(UserPointerValue<void> buffer) {
+MojoResult Core::UnmapBuffer(UserPointer<void> buffer) {
   base::AutoLock locker(mapping_table_lock_);
-  return mapping_table_.RemoveMapping(buffer.GetValue());
+  return mapping_table_.RemoveMapping(buffer.GetPointerValue());
 }
 
 // Note: We allow |handles| to repeat the same handle multiple times, since