[heap] Remove NewSpace::set_top and the corresponding hack

src/objects.cc

Index: src/objects.cc
diff --git a/src/objects.cc b/src/objects.cc
index b6c40d67fe8a64c13d4f7b44779e47281956a306..08383030d8ea63793ffd24ab057cb6df42363caa 100644
--- a/src/objects.cc
+++ b/src/objects.cc
@@ -9928,16 +9928,9 @@ Handle<String> SeqString::Truncate(Handle<SeqString> string, int new_length) {
   DCHECK_OBJECT_ALIGNED(start_of_string + new_size);
 
   Heap* heap = string->GetHeap();
-  NewSpace* newspace = heap->new_space();
-  if (newspace->Contains(start_of_string) &&
-      newspace->top() == start_of_string + old_size) {
-    // Last allocated object in new space.  Simply lower allocation top.
-    newspace->set_top(start_of_string + new_size);
-  } else {
-    // Sizes are pointer size aligned, so that we can use filler objects
-    // that are a multiple of pointer size.
-    heap->CreateFillerObjectAt(start_of_string + new_size, delta);
-  }
+  // Sizes are pointer size aligned, so that we can use filler objects
+  // that are a multiple of pointer size.
+  heap->CreateFillerObjectAt(start_of_string + new_size, delta);
   heap->AdjustLiveBytes(*string, -delta, Heap::CONCURRENT_TO_SWEEPER);
 
   // We are storing the new length using release store after creating a filler