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The genereated Ogg/Opus files don't seem to play in Chromium (which uses
ffmpeg) while they do VLC or Gstreamer. Turns out we have the wrong flag
set on the OpusTags packet and a wrong size written for one of the strings.

BUG=none
TEST=play the generated file on Chromium

Review URL: https://codereview.chromium.org/1640703002

Cr-Commit-Position: refs/heads/master@{#371778}