Make Response::body return v8-extra based stream behind flag

third_party/WebKit/Source/core/streams/UnderlyingSourceBase.h

 // Copyright 2016 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 UnderlyingSourceBase_h
 #define UnderlyingSourceBase_h
 
 #include "bindings/core/v8/ActiveScriptWrappable.h"
 #include "bindings/core/v8/ScriptPromise.h"
 #include "bindings/core/v8/ScriptState.h"
@@ skipping 21 matching lines @@
 
     virtual ScriptPromise pull(ScriptState*);
 
     ScriptPromise cancelWrapper(ScriptState*, ScriptValue reason);
     virtual ScriptPromise cancel(ScriptState*, ScriptValue reason);
 
     void notifyLockAcquired();
     void notifyLockReleased();
 
     // ActiveScriptWrappable
-    bool hasPendingActivity() const final;
+    bool hasPendingActivity() const;
 
     // ActiveDOMObject
     void stop() override;
 
 protected:
     explicit UnderlyingSourceBase(ScriptState* scriptState)
         : ActiveScriptWrappable(this)
         , ActiveDOMObject(scriptState->getExecutionContext())
     {
         this->suspendIfNeeded();
     }
 
     ReadableStreamController* controller() const { return m_controller; }
 
 private:
     Member<ReadableStreamController> m_controller;
     bool m_isStreamLocked = false;
 };
 
 } // namespace blink
 
 #endif // UnderlyingSourceBase_h