Revert of media::FFmpegVideoDecoder: remove ctor param |task_runner| and use a ThreadChecker instead.

Revert of media::FFmpegVideoDecoder: remove ctor param |task_runner| and use a ThreadChecker instead. (patchset #1 id:20001 of https://codereview.chromium.org/1456993004/ )

Reason for revert:
Broke compilation in a Mac bot
(https://chromegw.corp.google.com/i/official.desktop.continuous/builders/mac%20trunk/builds/20908)
(I wonder why the CQ passed)
http://crbug.com/558587

Original issue's description:
> media::FFmpegVideoDecoder: remove ctor param |task_runner| and use a ThreadChecker instead.
>
> This CL removed the |task_runner| passed on construction to
> FFmpegVideoDecoder since: "... is used in just one location
> for PostTask()ing. The class itself is otherwise mostly single
> threaded (except perhaps construction), and passing a
> |task_runner| as ctor argument is confusingly indicating the
> opposite. A |thread_checker_| basically tells that a(ll) methods
> are run in a given thread, whichever this is."
> (from discussion in http://crrev.com/1438233003, which has
> sadly been reverted for unrelated issues, and is going to be
> relanded as http://crrev.com/1447343004).
>
> Removal of this |task_runner| is also necessary for using these
> two classes from WebRtc, see the linekd bug.
>
> BUG=554737
>
> TEST=This is a tiny change, all unittest and content_browsertests should cover it.
>
> Committed: https://crrev.com/97e43b3e9b64da825f925a047b8d91e75260fd82
> Cr-Commit-Position: refs/heads/master@{#360628}

TBR=dalecurtis@chromium.org,chcunningham@chromium.org,bbudge@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=554737
Committed: https://crrev.com/506a31cfd353a555fbc92b8c642a01cc7675088d
Cr-Commit-Position: refs/heads/master@{#360649}

[mcasas, 2015-11-19 20:09:48]

Created Revert of media::FFmpegVideoDecoder: remove ctor param |task_runner| and
use a ThreadChecker instead.