Need to update last_slot when unprovisioned_bytes && discard.

base/allocator/partition_allocator/partition_alloc.cc

Index: base/allocator/partition_allocator/partition_alloc.cc
diff --git a/base/allocator/partition_allocator/partition_alloc.cc b/base/allocator/partition_allocator/partition_alloc.cc
index 1aa321f70c0fd12ee1c11953c7ecc821ec6103d9..7541eed3ce355b1ff6349fce3ab4eeaa576c423a 100644
--- a/base/allocator/partition_allocator/partition_alloc.cc
+++ b/base/allocator/partition_allocator/partition_alloc.cc
@@ -1162,6 +1162,7 @@ static size_t PartitionPurgePage(PartitionPage* page, bool discard) {
       *entry_ptr = PartitionFreelistMask(entry);
       entry_ptr = reinterpret_cast<PartitionFreelistEntry**>(entry);
       num_new_entries++;
+      last_slot = slotIndex;
     }
     // Terminate the freelist chain.
     *entry_ptr = nullptr;