Add mistagging-readbarrier to weak cell

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 332d1ed92c42f4e3f75f759cc9d2d7ce7f76ecd4..789d2936b6a7f9507e22e45bf776f766c24d7aff 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9714,7 +9714,8 @@ class PropertyCell: public Cell {
 
 class WeakCell : public HeapObject {
  public:
-  inline Object* value() const;
+  inline Object* value();
+  inline Object* ValueNoReadBarrier();
 
   // This should not be called by anyone except GC.
   inline void clear();
@@ -9732,11 +9733,14 @@ class WeakCell : public HeapObject {
   DECLARE_VERIFIER(WeakCell)
 
   // Layout description.
-  static const int kValueOffset = HeapObject::kHeaderSize;
-  static const int kNextOffset = kValueOffset + kPointerSize;
+  static const int kValueOffsetDontForgetTheReadBarrier =
+      HeapObject::kHeaderSize;
+  static const int kNextOffset =
+      kValueOffsetDontForgetTheReadBarrier + kPointerSize;
   static const int kSize = kNextOffset + kPointerSize;
 
-  typedef FixedBodyDescriptor<kValueOffset, kSize, kSize> BodyDescriptor;
+  typedef FixedBodyDescriptor<kValueOffsetDontForgetTheReadBarrier, kSize,
+                              kSize> BodyDescriptor;
 
  private:
   DISALLOW_IMPLICIT_CONSTRUCTORS(WeakCell);