Add "own" symbols support.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 80442b4608673f7a470a175d7bde310ebd1bec29..2bb47e80f547cd59ae32c6ffa692f481aea3a22a 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8954,6 +8954,9 @@ class Name: public HeapObject {
   // Conversion.
   inline bool AsArrayIndex(uint32_t* index);
 
+  // Whether name can only name own properties.
+  inline bool IsOwn();
+
   DECLARE_CAST(Name)
 
   DECLARE_PRINTER(Name)
@@ -9029,6 +9032,10 @@ class Symbol: public Name {
   // [is_private]: whether this is a private symbol.
   DECL_BOOLEAN_ACCESSORS(is_private)
 
+  // [is_own]: whether this is an own symbol, that is, only used to designate
+  // own properties of objects.
+  DECL_BOOLEAN_ACCESSORS(is_own)
+
   DECLARE_CAST(Symbol)
 
   // Dispatched behavior.
@@ -9044,6 +9051,7 @@ class Symbol: public Name {
 
  private:
   static const int kPrivateBit = 0;
+  static const int kOwnBit = 1;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(Symbol);
 };