Initialize pre-allocated fields of JSObject with undefined.

src/ia32/macro-assembler-ia32.h

Index: src/ia32/macro-assembler-ia32.h
diff --git a/src/ia32/macro-assembler-ia32.h b/src/ia32/macro-assembler-ia32.h
index ed66027b6eed0cc2db5a9fa15cef2307fa4cd98d..d1c0058586c95837b1e4ed7d820e7a7144f2c817 100644
--- a/src/ia32/macro-assembler-ia32.h
+++ b/src/ia32/macro-assembler-ia32.h
@@ -457,6 +457,13 @@ class MacroAssembler: public Assembler {
                  Register length,
                  Register scratch);
 
+  // Initialize fields with filler values.  Fields starting at |start_offset|
+  // not including end_offset are overwritten with the value in |filler|.  At
+  // the end the loop, |start_offset| takes the value of |end_offset|.
+  void InitializeFieldsWithFiller(Register start_offset,
+                                  Register end_offset,
+                                  Register filler);
+
   // ---------------------------------------------------------------------------
   // Support functions.