Bugfix: make dependent code field in AllocationSite weak.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 25c2210d5fa7ab96d2e9b0bf474f6c01765b4408..249b15fcc243337a3ac858ce793e32a9d83dcbfb 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8192,6 +8192,12 @@ class AllocationSite: public Struct {
   static const int kWeakNextOffset = kDependentCodeOffset + kPointerSize;
   static const int kSize = kWeakNextOffset + kPointerSize;
 
+  // During mark compact we need to take special care for the dependent code
+  // field.
+  static const int kPointerFieldsBeginOffset = kTransitionInfoOffset;
+  static const int kPointerFieldsEndOffset = kDependentCodeOffset;
+
+  // For other visitors, use the fixed body descriptor below.
   typedef FixedBodyDescriptor<HeapObject::kHeaderSize,
                               kDependentCodeOffset + kPointerSize,
                               kSize> BodyDescriptor;