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

src/objects.h

Index: src/objects.h
diff --git a/src/objects.h b/src/objects.h
index f7cf61fc8a38e53401f9786e718e681e5ff33c86..b44fe18df26da1f605e3a82dff19ba914e2bde6d 100644
--- a/src/objects.h
+++ b/src/objects.h
@@ -9680,6 +9680,10 @@ class Symbol: public Name {
   // a load.
   DECL_BOOLEAN_ACCESSORS(is_well_known_symbol)
 
+  // [is_public]: Whether this is a symbol created by Symbol.for. Calling
+  // Symbol.keyFor on such a symbol simply needs to return the attached name.
+  DECL_BOOLEAN_ACCESSORS(is_public)
+
   DECLARE_CAST(Symbol)
 
   // Dispatched behavior.
@@ -9694,6 +9698,7 @@ class Symbol: public Name {
   // Flags layout.
   static const int kPrivateBit = 0;
   static const int kWellKnownSymbolBit = 1;
+  static const int kPublicBit = 2;
 
   typedef FixedBodyDescriptor<kNameOffset, kFlagsOffset, kSize> BodyDescriptor;