[strong] Introduce strong bit

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 54375305bced4c8846dd5816ba333df065d9821b..2b722c078a038dba6bcf22005db395b3a38101c5 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -5900,7 +5900,8 @@ class Map: public HeapObject {
   class Deprecated : public BitField<bool, 23, 1> {};
   class IsUnstable : public BitField<bool, 24, 1> {};
   class IsMigrationTarget : public BitField<bool, 25, 1> {};
-  // Bits 26 and 27 are free.
+  class IsStrong : public BitField<bool, 26, 1> {};

[arv (Not doing code reviews), 2015/05/12 15:00:30]

Can you change this to use kNext instead?

+  // Bit 27 is free.
 
   // Keep this bit field at the very end for better code in
   // Builtins::kJSConstructStubGeneric stub.
@@ -5978,6 +5979,8 @@ class Map: public HeapObject {
     return ((1 << kIsObserved) & bit_field()) != 0;
   }
 
+  inline void set_is_strong(bool value);
+  inline bool is_strong();
   inline void set_is_extensible(bool value);
   inline bool is_extensible();
   inline void set_is_prototype_map(bool value);