Revert of Internalize strings in-place

src/heap/mark-compact.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/heap/mark-compact.h"
 
 #include "src/base/atomicops.h"
 #include "src/base/bits.h"
 #include "src/base/sys-info.h"
 #include "src/code-stubs.h"
@@ skipping 1419 matching lines @@
   }
 
   void VisitPointers(Object** start, Object** end) override {
     // Visit all HeapObject pointers in [start, end).
     MarkCompactCollector* collector = heap_->mark_compact_collector();
     for (Object** p = start; p < end; p++) {
       Object* o = *p;
       if (o->IsHeapObject()) {
         if (Marking::IsWhite(ObjectMarking::MarkBitFrom(HeapObject::cast(o)))) {
           if (finalize_external_strings) {
-            if (o->IsExternalString()) {
-              heap_->FinalizeExternalString(String::cast(*p));
-            } else {
-              // The original external string may have been internalized.
-              DCHECK(o->IsThinString());
-            }
+            DCHECK(o->IsExternalString());
+            heap_->FinalizeExternalString(String::cast(*p));
           } else {
             pointers_removed_++;
           }
           // Set the entry to the_hole_value (as deleted).
           *p = heap_->the_hole_value();
         } else if (record_slots) {
           // StringTable contains only old space strings.
           DCHECK(!heap_->InNewSpace(o));
           collector->RecordSlot(table_, p, o);
         }
@@ skipping 2630 matching lines @@
       // The target is always in old space, we don't have to record the slot in
       // the old-to-new remembered set.
       DCHECK(!heap()->InNewSpace(target));
       RecordRelocSlot(host, &rinfo, target);
     }
   }
 }
 
 }  // namespace internal
 }  // namespace v8