Make sure the heap is in consistent state when calling allocation observers.

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index 0703966dcda59f9100a90d77277b7322d635c9b2..7cfa4c294909fcb5bd3db25056d3947359983b2c 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -2444,7 +2444,6 @@ HeapObject* FreeList::Allocate(int size_in_bytes) {
   int new_node_size = 0;
   FreeSpace* new_node = FindNodeFor(size_in_bytes, &new_node_size);
   if (new_node == nullptr) return nullptr;
-  owner_->AllocationStep(new_node->address(), size_in_bytes);
 
   int bytes_left = new_node_size - size_in_bytes;
   DCHECK(bytes_left >= 0);
@@ -2490,6 +2489,8 @@ HeapObject* FreeList::Allocate(int size_in_bytes) {
                            new_node->address() + new_node_size);
   }
 
+  owner_->AllocationStep(new_node->address(), size_in_bytes);
+
   return new_node;
 }