fetch: Align RequestInit's |headers| with the spec.

third_party/WebKit/Source/modules/fetch/RequestInit.h

 // Copyright 2014 The Chromium 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 RequestInit_h
 #define RequestInit_h
 
+#include "bindings/modules/v8/ByteStringSequenceSequenceOrByteStringByteStringRecordOrHeaders.h"
+#include "modules/fetch/Headers.h"
 #include "platform/heap/Handle.h"
 #include "platform/network/EncodedFormData.h"
 #include "platform/weborigin/Referrer.h"
 #include "platform/wtf/RefPtr.h"
 #include "platform/wtf/text/WTFString.h"
 
 namespace blink {
 
 class BytesConsumer;
 class Dictionary;
 class ExecutionContext;
 class ExceptionState;
-class Headers;
 
 // FIXME: Use IDL dictionary instead of this class.
 class RequestInit {
   STACK_ALLOCATED();
 
  public:
   explicit RequestInit(ExecutionContext*, const Dictionary&, ExceptionState&);
 
   String method;
-  Member<Headers> headers;
+  HeadersInit headers;
   String content_type;
   Member<BytesConsumer> body;
   Referrer referrer;
   String mode;
   String credentials;
   String cache;
   String redirect;
   String integrity;
   RefPtr<EncodedFormData> attached_credential;
   // True if any members in RequestInit are set and hence the referrer member
   // should be used in the Request constructor.
   bool are_any_members_set;
 };
 
 }  // namespace blink
 
 #endif  // RequestInit_h