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After a seek, both audio and video decoder job will start prerolling.
However, audio decoder job can finish prerolling much earlier than video decoder job.
As a result, audio will start playing before video starts.
And we will see some fast motion when video try to catch up with audio after it starts rendering.

internal b/17527385

Review URL: https://codereview.chromium.org/596093002

Cr-Commit-Position: refs/heads/master@{#296607}