Remove mojo::DemuxerStreamObserver.

Remove mojo::DemuxerStreamObserver.

The observer is running on another pipe which could cause race conditions and
make the implementation unnecessarily complex. This CL drops the observer and
merges the config update into the main mojo::DemuxerStream interface.

BUG=460154
TEST=No DCHECK fires and playback works. mojo:media_test passes.

Committed: https://crrev.com/e19d54eeef0840602b27040c3ad64030da9f108c
Cr-Commit-Position: refs/heads/master@{#319704}

[xhwang, 2015-03-07 01:27:10]

xhwang@chromium.org changed reviewers:
+ dalecurtis@chromium.org

[xhwang, 2015-03-07 01:27:10]

dalecurtis: PTAL

mikhal/jamesr: FYI

[DaleCurtis, 2015-03-09 18:19:55]

lgtm

[xhwang, 2015-03-09 18:34:51]

The CQ bit was checked by xhwang@chromium.org

[commit-bot: I haz the power, 2015-03-09 18:35:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/981423002/1

[commit-bot: I haz the power, 2015-03-09 19:17:53]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-09 19:18:19]

Patchset 1 (id:??) landed as
https://crrev.com/e19d54eeef0840602b27040c3ad64030da9f108c
Cr-Commit-Position: refs/heads/master@{#319704}