Inobject slack tracking is done on a per-closure basis instead of per-shared info basis.

src/arm64/macro-assembler-arm64.h

Index: src/arm64/macro-assembler-arm64.h
diff --git a/src/arm64/macro-assembler-arm64.h b/src/arm64/macro-assembler-arm64.h
index c9ebab304bee6c928fdce2cea76ae21cddcb3e78..f72a788e9a228c1a24016d13611055a62a568a60 100644
--- a/src/arm64/macro-assembler-arm64.h
+++ b/src/arm64/macro-assembler-arm64.h
@@ -842,10 +842,15 @@ class MacroAssembler : public Assembler {
   void NumberOfOwnDescriptors(Register dst, Register map);
 
   template<typename Field>
-  void DecodeField(Register reg) {
+  void DecodeField(Register dst, Register src) {
     static const uint64_t shift = Field::kShift;
     static const uint64_t setbits = CountSetBits(Field::kMask, 32);
-    Ubfx(reg, reg, shift, setbits);
+    Ubfx(dst, src, shift, setbits);
+  }
+
+  template<typename Field>
+  void DecodeField(Register reg) {
+    DecodeField<Field>(reg, reg);
   }
 
   // ---- SMI and Number Utilities ----