CopyOnWriteBuffer::SetSize to smaller size memcpy less.
CopyOnWriteBuffer::SetSize to smaller size memcpy less.

R=kwiberg@webrtc.org

Review URL: https://codereview.webrtc.org/2317283004 .

Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#14166}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 291cd8fac37a2efa026713d198d653bc083881db

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