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

The observer might want to lookup something in the heap, e.g. code objects
it has in the call stack.

BUG=v8:4959
LOG=N

Review-Url: https://codereview.chromium.org/1948893002
Cr-Commit-Position: refs/heads/master@{#36027}

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;
 }