[Fetch] Response.clone should tee the body stream.

Source/modules/serviceworkers/RespondWithObserver.cpp

Index: Source/modules/serviceworkers/RespondWithObserver.cpp
diff --git a/Source/modules/serviceworkers/RespondWithObserver.cpp b/Source/modules/serviceworkers/RespondWithObserver.cpp
index 72a2e545db0c64c8bdf5919f0695f42f0b89cf58..d5dc9e0e9faaa6161ecacf38d5b41db2f2aed210 100644
--- a/Source/modules/serviceworkers/RespondWithObserver.cpp
+++ b/Source/modules/serviceworkers/RespondWithObserver.cpp
@@ -201,7 +201,7 @@ void RespondWithObserver::responseWasFulfilled(const ScriptValue& value)
         responseWasRejected();
         return;
     }
-    response->setBodyUsed();
+    response->lockBody(Body::PassBody);
     if (BodyStreamBuffer* buffer = response->internalBuffer()) {
         if (buffer == response->buffer() && response->streamAccessed())
             buffer = response->createDrainingStream();