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

Source/modules/fetch/FetchBlobDataConsumerHandle.cpp

 // 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.
 
 #include "config.h"
 #include "modules/fetch/FetchBlobDataConsumerHandle.h"
 
 #include "core/dom/ActiveDOMObject.h"
 #include "core/dom/CrossThreadTask.h"
 #include "core/dom/ExecutionContext.h"
@@ skipping 345 matching lines @@
             , m_reader(reader)
             , m_notifier(client) { }
         ~ReaderImpl() override { }
 
         Result read(void* data, size_t size, Flags flags, size_t* readSize) override
         {
             if (m_readerContext->drained())
                 return Done;
             m_readerContext->ensureStartLoader();
             Result r = m_reader->read(data, size, flags, readSize);
-            if (r != ShouldWait) {
+            if (r != ShouldWait && !(r == Ok && *readSize == 0)) {
                 // We read non-empty data, so we cannot use the blob data
                 // handle which represents the whole data.
                 m_readerContext->clearBlobDataHandleForDrain();
             }
             return r;
         }
 
         Result beginRead(const void** buffer, Flags flags, size_t* available) override
         {
             if (m_readerContext->drained())
                 return Done;
             m_readerContext->ensureStartLoader();
             Result r = m_reader->beginRead(buffer, flags, available);
-            if (r != ShouldWait) {
+            if (r != ShouldWait && !(r == Ok && *available == 0)) {
                 // We read non-empty data, so we cannot use the blob data
                 // handle which represents the whole data.
                 m_readerContext->clearBlobDataHandleForDrain();
             }
             return r;
         }
 
         Result endRead(size_t readSize) override
         {
             return m_reader->endRead(readSize);
@@ skipping 80 matching lines @@
 
     return adoptPtr(new FetchBlobDataConsumerHandle(executionContext, blobDataHandle, new DefaultLoaderFactory));
 }
 
 FetchDataConsumerHandle::Reader* FetchBlobDataConsumerHandle::obtainReaderInternal(Client* client)
 {
     return m_readerContext->obtainReader(client).leakPtr();
 }
 
 } // namespace blink