Allow specifying base offset when constructing Keyed hydrogen instructions

src/elements-kind.cc

Index: src/elements-kind.cc
diff --git a/src/elements-kind.cc b/src/elements-kind.cc
index adab39679d1f90e637acf04d8347b50706da4075..d585eaa4dd3c7a01e146812be2ab0700edfac4ca 100644
--- a/src/elements-kind.cc
+++ b/src/elements-kind.cc
@@ -51,6 +51,12 @@ int ElementsKindToShiftSize(ElementsKind elements_kind) {
 }
 
 
+int GetDefaultHeaderSizeForElementsKind(ElementsKind elements_kind) {
+  return IsExternalArrayElementsKind(elements_kind)
+      ? 0 : (FixedArray::kHeaderSize - kSmiTagSize);
+}
+
+
 const char* ElementsKindToString(ElementsKind kind) {
   ElementsAccessor* accessor = ElementsAccessor::ForKind(kind);
   return accessor->name();