Introduce FieldIndex to unify and abstract property/field offset

src/objects-printer.cc

Index: src/objects-printer.cc
diff --git a/src/objects-printer.cc b/src/objects-printer.cc
index c51c04e3fe223e01367dcb8843405338039e9623..43664aa9bf5236e99b7cc5174bbc069f0e1a7ecc 100644
--- a/src/objects-printer.cc
+++ b/src/objects-printer.cc
@@ -232,9 +232,15 @@ void JSObject::PrintProperties(FILE* out) {
       PrintF(out, ": ");
       switch (descs->GetType(i)) {
         case FIELD: {
-          int index = descs->GetFieldIndex(i);
+          FieldIndex index = FieldIndex::ForDescriptor(map(), i);
           RawFastPropertyAt(index)->ShortPrint(out);
-          PrintF(out, " (field at offset %d)\n", index);
+          if (index.is_inobject()) {
+            PrintF(out, " (inobject field at index %d)\n",
+                   index.property_index());
+          } else {
+            PrintF(out, " (out-of-object field at index %d)\n",
+                   index.property_index());
+          }
           break;
         }
         case CONSTANT: