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

src/heap/heap.h

Index: src/heap/heap.h
diff --git a/src/heap/heap.h b/src/heap/heap.h
index 1eb9c7c8a8458d52077fe1b4ebb0dea8f870d78d..c2370e72a2d72badc00ce840397fdb2e978271ce 100644
--- a/src/heap/heap.h
+++ b/src/heap/heap.h
@@ -37,7 +37,6 @@ namespace internal {
   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)                      \