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

Source/modules/fetch/DataConsumerHandleUtil.h

 // Copyright 2015 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 DataConsumerHandleUtil_h
 #define DataConsumerHandleUtil_h
 
 #include "modules/ModulesExport.h"
 #include "modules/fetch/FetchDataConsumerHandle.h"
 #include "public/platform/WebDataConsumerHandle.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/WeakPtr.h"
 
 namespace blink {
 
 // Returns a handle that returns ShouldWait for read / beginRead and
 // UnexpectedError for endRead.
 MODULES_EXPORT PassOwnPtr<WebDataConsumerHandle> createWaitingDataConsumerHandle();
 
 // Returns a handle that returns Done for read / beginRead and
 // UnexpectedError for endRead.
 MODULES_EXPORT PassOwnPtr<WebDataConsumerHandle> createDoneDataConsumerHandle();
 
 // Returns a handle that returns UnexpectedError for read / beginRead /
 // endRead.
 MODULES_EXPORT PassOwnPtr<WebDataConsumerHandle> createUnexpectedErrorDataConsumerHandle();
 
 // Returns a FetchDataConsumerHandle that wraps WebDataConsumerHandle.
-PassOwnPtr<FetchDataConsumerHandle> createFetchDataConsumerHandleFromWebHandle(PassOwnPtr<WebDataConsumerHandle>);
+MODULES_EXPORT PassOwnPtr<FetchDataConsumerHandle> createFetchDataConsumerHandleFromWebHandle(PassOwnPtr<WebDataConsumerHandle>);
 
 // A helper class to call Client::didGetReadable() asynchronously after
 // Reader creation.
 // NotifyOnReaderCreationHelper must be owned by a reader and
 // |client| must be the client of the reader.
 class NotifyOnReaderCreationHelper final {
 public:
     NotifyOnReaderCreationHelper(WebDataConsumerHandle::Client* /* client */);
 
 private:
     void notify(WebDataConsumerHandle::Client*);
 
     WeakPtrFactory<NotifyOnReaderCreationHelper> m_factory;
 };
 
 } // namespace blink
 
 #endif // DataConsumerHandleUtil_h