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

third_party/WebKit/Source/modules/fetch/Headers.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 Headers_h
 #define Headers_h
 
 #include "bindings/core/v8/Iterable.h"
 #include "bindings/core/v8/ScriptState.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "modules/ModulesExport.h"
 #include "modules/fetch/FetchHeaderList.h"
 #include "platform/wtf/Forward.h"
 
 namespace blink {
 
 class ByteStringSequenceSequenceOrByteStringByteStringRecordOrHeaders;
 class ExceptionState;
 
+using HeadersInit =
+    ByteStringSequenceSequenceOrByteStringByteStringRecordOrHeaders;
+
 // http://fetch.spec.whatwg.org/#headers-class
 class MODULES_EXPORT Headers final : public GarbageCollected<Headers>,
                                      public ScriptWrappable,
                                      public PairIterable<String, String> {
   DEFINE_WRAPPERTYPEINFO();
 
  public:
   enum Guard {
     kImmutableGuard,
     kRequestGuard,
     kRequestNoCORSGuard,
     kResponseGuard,
     kNoneGuard
   };
 
   static Headers* Create(ExceptionState&);
-  static Headers* Create(
-      const ByteStringSequenceSequenceOrByteStringByteStringRecordOrHeaders&,
-      ExceptionState&);
+  static Headers* Create(const HeadersInit&, ExceptionState&);
 
   // Shares the FetchHeaderList. Called when creating a Request or Response.
   static Headers* Create(FetchHeaderList*);
 
   Headers* Clone() const;
 
   // Headers.idl implementation.
   void append(const String& name, const String& value, ExceptionState&);
   void remove(const String& key, ExceptionState&);
   String get(const String& key, ExceptionState&);
   bool has(const String& key, ExceptionState&);
   void set(const String& key, const String& value, ExceptionState&);
 
   void SetGuard(Guard guard) { guard_ = guard; }
   Guard GetGuard() const { return guard_; }
 
   // These methods should only be called when size() would return 0.
   void FillWith(const Headers*, ExceptionState&);
-  void FillWith(const Vector<Vector<String>>&, ExceptionState&);
-  void FillWith(const Vector<std::pair<String, String>>&, ExceptionState&);
+  void FillWith(const HeadersInit&, ExceptionState&);
 
   FetchHeaderList* HeaderList() const { return header_list_; }
   DECLARE_TRACE();
 
  private:
   Headers();
   // Shares the FetchHeaderList. Called when creating a Request or Response.
   explicit Headers(FetchHeaderList*);
 
+  // These methods should only be called when size() would return 0.
+  void FillWith(const Vector<Vector<String>>&, ExceptionState&);
+  void FillWith(const Vector<std::pair<String, String>>&, ExceptionState&);
+
   Member<FetchHeaderList> header_list_;
   Guard guard_;
 
   IterationSource* StartIteration(ScriptState*, ExceptionState&) override;
 };
 
 }  // namespace blink
 
 #endif  // Headers_h