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

Source/modules/fetch/DataConsumerTee.cpp

Index: Source/modules/fetch/DataConsumerTee.cpp
diff --git a/Source/modules/fetch/DataConsumerTee.cpp b/Source/modules/fetch/DataConsumerTee.cpp
index 72b6750742dc59cf7b1b334dee2d05a577171f86..dfa4d4343fdf9a4e592f600f83c77f86e7a6d5e0 100644
--- a/Source/modules/fetch/DataConsumerTee.cpp
+++ b/Source/modules/fetch/DataConsumerTee.cpp
@@ -7,6 +7,8 @@
 
 #include "core/dom/ActiveDOMObject.h"
 #include "core/dom/ExecutionContext.h"
+#include "modules/fetch/DataConsumerHandleUtil.h"
+#include "modules/fetch/FetchBlobDataConsumerHandle.h"
 #include "platform/Task.h"
 #include "platform/ThreadSafeFunctional.h"
 #include "platform/heap/Handle.h"
@@ -422,4 +424,20 @@ void DataConsumerTee::create(ExecutionContext* executionContext, PassOwnPtr<WebD
     *dest2 = adoptPtr(new DestinationHandle(DestinationContext::Proxy::create(context2, tracker)));
 }
 
+void DataConsumerTee::create(ExecutionContext* executionContext, PassOwnPtr<FetchDataConsumerHandle> src, OwnPtr<FetchDataConsumerHandle>* dest1, OwnPtr<FetchDataConsumerHandle>* dest2)
+{
+    RefPtr<BlobDataHandle> blobDataHandle = src->obtainReader(nullptr)->drainAsBlobDataHandle(FetchDataConsumerHandle::Reader::AllowBlobWithInvalidSize);
+    if (blobDataHandle) {
+        *dest1 = FetchBlobDataConsumerHandle::create(executionContext, blobDataHandle);
+        *dest2 = FetchBlobDataConsumerHandle::create(executionContext, blobDataHandle);
+        return;
+    }
+
+    OwnPtr<WebDataConsumerHandle> webDest1, webDest2;
+    DataConsumerTee::create(executionContext, static_cast<PassOwnPtr<WebDataConsumerHandle>>(src), &webDest1, &webDest2);
+    *dest1 = createFetchDataConsumerHandleFromWebHandle(webDest1.release());
+    *dest2 = createFetchDataConsumerHandleFromWebHandle(webDest2.release());
+    return;
+}
+
 } // namespace blink