[runtime] Replace hidden_string with a 0-hash-code private symbol

src/heap/heap.h

 // 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.
 
 #ifndef V8_HEAP_HEAP_H_
 #define V8_HEAP_HEAP_H_
 
 #include <cmath>
 #include <map>
 
@@ skipping 19 matching lines @@
   V(Map, one_pointer_filler_map, OnePointerFillerMap)                          \
   V(Map, two_pointer_filler_map, TwoPointerFillerMap)                          \
   /* Cluster the most popular ones in a few cache lines here at the top.    */ \
   V(Smi, store_buffer_top, StoreBufferTop)                                     \
   V(Oddball, undefined_value, UndefinedValue)                                  \
   V(Oddball, the_hole_value, TheHoleValue)                                     \
   V(Oddball, null_value, NullValue)                                            \
   V(Oddball, true_value, TrueValue)                                            \
   V(Oddball, false_value, FalseValue)                                          \
   V(String, empty_string, empty_string)                                        \
-  V(String, hidden_string, hidden_string)                                      \
   V(Oddball, uninitialized_value, UninitializedValue)                          \
   V(Map, cell_map, CellMap)                                                    \
   V(Map, global_property_cell_map, GlobalPropertyCellMap)                      \
   V(Map, shared_function_info_map, SharedFunctionInfoMap)                      \
   V(Map, meta_map, MetaMap)                                                    \
   V(Map, heap_number_map, HeapNumberMap)                                       \
   V(Map, mutable_heap_number_map, MutableHeapNumberMap)                        \
   V(Map, float32x4_map, Float32x4Map)                                          \
   V(Map, int32x4_map, Int32x4Map)                                              \
   V(Map, uint32x4_map, Uint32x4Map)                                            \
@@ skipping 2575 matching lines @@
   friend class LargeObjectSpace;
   friend class NewSpace;
   friend class PagedSpace;
   DISALLOW_COPY_AND_ASSIGN(AllocationObserver);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_HEAP_HEAP_H_