`ExpandHeap_lock` is used in the `expand` method in young-gen and old-gen of Serial GC. Inspecting the call hierarchy, one can see it's called either inside a safepoint or holding `Heap_lock` (i.e. `assert_locked_or_safepoint(Heap_lock)` holds).

Therefore, `ExpandHeap_lock` can be removed in Serial GC code.