Fix CopyOnWriteBuffer::cdata to return const pointer to prevent unexpected data modification.
Fix CopyOnWriteBuffer::cdata to return const pointer
to prevent unexpected data modification.

Review-Url: https://codereview.webrtc.org/2189013002
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13583}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8a0b1287cde0248843e649cc9e01f7d861649d34

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