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I'm currently refactoring some code that uses DecoderBuffer to use
DecoderBufferBase instead (so it can run in a cma backend).  It also
depends on DecoderBuffer::set_timestamp, so we need an equivalent
in DecoderBufferBase.

BUG=

Review URL: https://codereview.chromium.org/1145163005

Cr-Commit-Position: refs/heads/master@{#333138}