Do not set |Body::ReadableStreamSource::m_bodyStreamBuffer| in close()/error() if initially null
Do not set |Body::ReadableStreamSource::m_bodyStreamBuffer| in close()/error() if initially null

createDrainingStream() should return nullptr if body is null, but the current implementation returns non-null after close()/error().

BUG=480746

Review URL: https://codereview.chromium.org/1221333014

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@198458 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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