Remove final from rtc::Buffer.
Remove final from rtc::Buffer.

With it removed, you can now use it with scoped_refptr by wrapping it in
an rtc::RefCountedObject<rtc::Buffer>.

BUG=

Review URL: https://codereview.webrtc.org/1414053003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#10386}

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