v8::Private::ForApi should be context-independent.

src/builtins/builtins.h

 // Copyright 2011 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_BUILTINS_BUILTINS_H_
 #define V8_BUILTINS_BUILTINS_H_
 
 #include "src/base/flags.h"
 #include "src/handles.h"
 
@@ skipping 658 matching lines @@
   TFJ(StringPrototypeValueOf, 0)                                              \
   /* ES6 #sec-string.prototype-@@iterator */                                  \
   TFJ(StringPrototypeIterator, 0)                                             \
                                                                               \
   /* StringIterator */                                                        \
   TFJ(StringIteratorPrototypeNext, 0)                                         \
                                                                               \
   /* Symbol */                                                                \
   CPP(SymbolConstructor)                                                      \
   CPP(SymbolConstructor_ConstructStub)                                        \
+  /* ES6 section 19.4.2.1 Symbol.for */                                       \
+  CPP(SymbolFor)                                                              \
+  /* ES6 section 19.4.2.5 Symbol.keyFor */                                    \
+  CPP(SymbolKeyFor)                                                           \
   /* ES6 section 19.4.3.4 Symbol.prototype [ @@toPrimitive ] ( hint ) */      \
   TFJ(SymbolPrototypeToPrimitive, 1)                                          \
   /* ES6 section 19.4.3.2 Symbol.prototype.toString ( ) */                    \
   TFJ(SymbolPrototypeToString, 0)                                             \
   /* ES6 section 19.4.3.3 Symbol.prototype.valueOf ( ) */                     \
   TFJ(SymbolPrototypeValueOf, 0)                                              \
                                                                               \
   /* TypedArray */                                                            \
   CPP(TypedArrayPrototypeBuffer)                                              \
   /* ES6 section 22.2.3.2 get %TypedArray%.prototype.byteLength */            \
@@ skipping 153 matching lines @@
 
   friend class Isolate;
 
   DISALLOW_COPY_AND_ASSIGN(Builtins);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_BUILTINS_BUILTINS_H_