Transcoder: Add TrackTranscoder benchmarking.

Adds a benchmarking test that can run TrackTranscoders
without getting samples directly from an extractor and
without writing the transcoded samples to a muxer.

Test: Running the benchmark.
Fixes: 152091444

Change-Id: Ia723f96b9033b1ec9b4502a1242e154bcdbfe9f5