Comment 2 for bug 537617

If we don't ship it, tech users will grab it from one of the PPAs, and worse, the vast majority of ubuntu users will just stop using Chromium the first time they hit a <video> and move to the upstream Google Chrome deb which ships the whole thing by default.

BTW, it's not a copy of ffmpeg per say, our system ffmpeg is not multi-threaded.
And Google is taking care of the security of that thing, along with upstream, hence the stack of patches in the tarball.