Small fix for a trace-file race condition.
Small fix for a trace-file race condition.

On the waterfall we were seeing identical buffers written to a trace-file
multiple times. This small fix eliminates the problem by cleaning up buffers
prior to recycling them.

BUG=
TEST=

Review URL: https://codereview.appspot.com/6213045

git-svn-id: db59699583a60be9a535cd09cdc9132301867226@811 15e8cca8-e42c-11de-a347-f34a4f72eb7d

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