[Interpreter] Add ForInPrepare runtime function which returns a ObjectTriple.

src/runtime/runtime-utils.h

 // Copyright 2014 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_RUNTIME_RUNTIME_UTILS_H_
 #define V8_RUNTIME_RUNTIME_UTILS_H_
 
 #include "src/runtime/runtime.h"
 
 namespace v8 {
@@ skipping 144 matching lines @@
          (reinterpret_cast<ObjectPair>(y) << 32);
 #elif defined(V8_TARGET_BIG_ENDIAN)
   return reinterpret_cast<uint32_t>(y) |
          (reinterpret_cast<ObjectPair>(x) << 32);
 #else
 #error Unknown endianness
 #endif
 }
 #endif
 
+
+// A mechanism to return a triple of Object pointers. In all calling
+// conventions, a struct of two pointers is returned in memory,
+// allocated by the caller, and passed as a pointer in a hidden first parameter.
+struct ObjectTriple {
+  Object* x;
+  Object* y;
+  Object* z;
+};
+
+static inline ObjectTriple MakeTriple(Object* x, Object* y, Object* z) {
+  ObjectTriple result = {x, y, z};
+  // ObjectTriple is assigned to a hidden first argument.
+  return result;
+}
+
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_RUNTIME_RUNTIME_UTILS_H_