Change BodyStreamBuffer to be FetchDataConsumerHandle-based and enable backpressure in Fetch API

Source/modules/fetch/Request.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 Request_h
 #define Request_h
 
 #include "bindings/core/v8/Dictionary.h"
 #include "bindings/core/v8/ScriptWrappable.h"
 #include "bindings/modules/v8/UnionTypesModules.h"
 #include "modules/ModulesExport.h"
 #include "modules/fetch/Body.h"
 #include "modules/fetch/FetchRequestData.h"
 #include "modules/fetch/Headers.h"
 #include "platform/heap/Handle.h"
 #include "platform/weborigin/KURL.h"
 #include "public/platform/WebURLRequest.h"
 #include "wtf/RefPtr.h"
 #include "wtf/text/WTFString.h"
 
 namespace blink {
 
-class BodyStreamBuffer;
+class BodyStreamBuffer2;
 class RequestInit;
 class WebServiceWorkerRequest;
 
 typedef RequestOrUSVString RequestInfo;
 
 class MODULES_EXPORT Request final : public Body {
     DEFINE_WRAPPERTYPEINFO();
 public:
     ~Request() override { }
 
@@ skipping 31 matching lines @@
     DECLARE_VIRTUAL_TRACE();
 
 private:
     Request(ExecutionContext*, FetchRequestData*);
     Request(ExecutionContext*, const WebServiceWorkerRequest&);
     Request(ExecutionContext*, FetchRequestData*, Headers*);
 
     static Request* createRequestWithRequestOrString(ScriptState*, Request*, const String&, const RequestInit&, ExceptionState&);
     void clearHeaderList();
 
-    PassRefPtr<BlobDataHandle> blobDataHandle() const override;
-    BodyStreamBuffer* buffer() const override;
+    BodyStreamBuffer2* buffer() const override;
     String mimeType() const override;
 
     const Member<FetchRequestData> m_request;
     const Member<Headers> m_headers;
 };
 
 } // namespace blink
 
 #endif // Request_h