[promises] PromiseResolveThenableJob: change then to be a JSReceiver

src/objects.h

 // Copyright 2015 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_OBJECTS_H_
 #define V8_OBJECTS_H_
 
 #include <iosfwd>
 #include <memory>
 
@@ skipping 6644 matching lines @@
   inline void InitializeBody(int object_size);
   DECLARE_CAST(Struct)
 };
 
 // A container struct to hold state required for
 // PromiseResolveThenableJob. {before, after}_debug_event could
 // potentially be undefined if the debugger is turned off.
 class PromiseContainer : public Struct {
  public:
   DECL_ACCESSORS(thenable, JSReceiver)
-  DECL_ACCESSORS(then, JSFunction)
+  DECL_ACCESSORS(then, JSReceiver)
   DECL_ACCESSORS(resolve, JSFunction)
   DECL_ACCESSORS(reject, JSFunction)
   DECL_ACCESSORS(before_debug_event, Object)
   DECL_ACCESSORS(after_debug_event, Object)
 
   static const int kThenableOffset = Struct::kHeaderSize;
   static const int kThenOffset = kThenableOffset + kPointerSize;
   static const int kResolveOffset = kThenOffset + kPointerSize;
   static const int kRejectOffset = kResolveOffset + kPointerSize;
   static const int kBeforeDebugEventOffset = kRejectOffset + kPointerSize;
@@ skipping 4722 matching lines @@
     }
     return value;
   }
 };
 
 
 }  // NOLINT, false-positive due to second-order macros.
 }  // NOLINT, false-positive due to second-order macros.
 
 #endif  // V8_OBJECTS_H_