Add "own" symbols support.

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index 0001eb5dd4c4a889a6ea7110efc92cbe8cb0f644..cbc7240f10662bebf45760d30165ebe88ffc7aff 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -8960,6 +8960,9 @@ 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)
@@ -9035,6 +9038,10 @@ 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.
@@ -9050,6 +9057,7 @@ class Symbol: public Name {
 
  private:
   static const int kPrivateBit = 0;
+  static const int kOwnBit = 1;
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(Symbol);
 };