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

Source/modules/fetch/FetchDataLoader.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 FetchDataLoader_h
 #define FetchDataLoader_h
 
 #include "core/dom/DOMArrayBuffer.h"
 #include "core/streams/Stream.h"
 #include "modules/ModulesExport.h"
 #include "modules/fetch/FetchDataConsumerHandle.h"
 #include "platform/blob/BlobData.h"
 #include "platform/heap/Handle.h"
 #include "wtf/Forward.h"
 
 namespace blink {
 
 // FetchDataLoader subclasses
 // 1. take a reader of FetchDataConsumerHandle |handle|,
 // 2. read all data, and
 // 3. call either didFetchDataLoaded...() on success or
 //    difFetchDataLoadFailed() otherwise
 //    on the thread where FetchDataLoader is created.
 //
 // - Client's methods can be called synchronously in start().
+// -- Specifically, if the first call by FetchDataLoader to read()/beginRead()
+//    returns values other than Ok or ShouldWait,
+//    Client's methods must be called synchronously in start().
 // - If FetchDataLoader::cancel() is called (or FetchDataLoader is garbage
 //   collected), Client's methods are not called anymore.
 // - FetchDataLoader takes the ownership of |handle|'s reader but not |handle|.
 class MODULES_EXPORT FetchDataLoader : public GarbageCollectedFinalized<FetchDataLoader> {
 public:
     class Client : public GarbageCollectedMixin {
     public:
         virtual ~Client() { }
 
         // The method corresponding to createLoaderAs... is called on success.
@@ skipping 34 matching lines @@
     virtual void start(FetchDataConsumerHandle* /* handle */, Client*) = 0;
 
     virtual void cancel() = 0;
 
     DEFINE_INLINE_VIRTUAL_TRACE() { }
 };
 
 } // namespace blink
 
 #endif // FetchDataLoader_h