Opus seek preroll was being treated as decoder delay when it really just means that for seeking to a time T, we need to decode packets starting from time "T - seek_preroll" and throw them away. Instead we were discarding time [T, T+seek_preroll] and shifting the timestamps backwards to accommodate the deletion. For pre-skip, we would assume the first packet seen by the decoder was the timestamp for which pre-skip should be applied. However, this is incorrect and pre-skip should always be dropped because timestamp zero represents the beginning of the pre-skip, which causes all future timestamps to be displaced by pre-skip duration. This patch fixes pre-skip and seek preroll for src= and pre-skip only for MSE. ChunkDemuxer needs to be modified to support preroll. src= is fixed by expanding the hacks in place for negative ogg timestamps to handle negative timestamps from ffmpeg in a more general manner. This patch also adds test cases for the seeking cases (one of which MSE can pass because the test clip has a codec delay > seek preroll). Since these tests would otherwise take some time to run, this also adds support for clockless hashed pipeline integration tests. BUG=509894 TEST=OpusAudioDecoder and OPUS PipelineIntegration tests pass w/o expectations modification. New trimming pipeline tests. New FFmpegDemuxerTest to verify audio/video timestamp sync. Review URL: https://codereview.chromium.org/1260193005 Cr-Commit-Position: refs/heads/master@{#342274}