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

Source/modules/fetch/Request.h

Index: Source/modules/fetch/Request.h
diff --git a/Source/modules/fetch/Request.h b/Source/modules/fetch/Request.h
index 30727ffa2bb97caf1a1288c95daa7f10f0c0577f..62c0cb5baaec88ab22ebc13e3560f554403c12e9 100644
--- a/Source/modules/fetch/Request.h
+++ b/Source/modules/fetch/Request.h
@@ -20,7 +20,7 @@
 
 namespace blink {
 
-class BodyStreamBuffer;
+class BodyStreamBuffer2;
 class RequestInit;
 class WebServiceWorkerRequest;
 
@@ -72,8 +72,7 @@ private:
     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;