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

src/arm/macro-assembler-arm.h

Index: src/arm/macro-assembler-arm.h
diff --git a/src/arm/macro-assembler-arm.h b/src/arm/macro-assembler-arm.h
index 9a2ee7a17d72133d04c2631bcf82e6f111f925f4..d49891926cada9a66a6661891459b6bbbdcb3591 100644
--- a/src/arm/macro-assembler-arm.h
+++ b/src/arm/macro-assembler-arm.h
@@ -1355,11 +1355,19 @@ 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 int shift = Field::kShift;
     static const int mask = Field::kMask >> shift;
-    mov(reg, Operand(reg, LSR, shift));
-    and_(reg, reg, Operand(mask));
+    static const int size = Field::kSize;
+    mov(dst, Operand(src, LSR, shift));
+    if (shift + size != 32) {
+      and_(dst, dst, Operand(mask));
+    }
+  }
+
+  template<typename Field>
+  void DecodeField(Register reg) {
+    DecodeField<Field>(reg, reg);
   }
 
   // Activation support.