Make DataSource::Stop() synchronous.

Make DataSource::Stop() synchronous.

In a lot of filters in media code, Stop() has been folded into the dtor.
This model doesn't apply directly to DataSource because DataSource::Stop()
is called by the FFmpegDemuxer, but the DataSource is owned by
WebMediaPlayerImpl.

TBR=gbillock@chromium.org
BUG=349211
TEST=All existing tests pass.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285479

[xhwang, 2014-07-24 20:03:26]

Here's another one. PTAL.

[scherkus (not reviewing), 2014-07-24 22:20:59]

lgtm

[xhwang, 2014-07-24 22:29:26]

The CQ bit was checked by xhwang@chromium.org

[xhwang, 2014-07-24 22:29:30]

The CQ bit was unchecked by xhwang@chromium.org

[xhwang, 2014-07-24 22:30:57]

TBRing gbillock@chromium.org for changes in chrome/utility/media_galleries/*

[xhwang, 2014-07-24 22:31:01]

The CQ bit was checked by xhwang@chromium.org

[commit-bot: I haz the power, 2014-07-24 22:33:52]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/xhwang@chromium.org/413243002/20001

[commit-bot: I haz the power, 2014-07-25 00:13:59]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[commit-bot: I haz the power, 2014-07-25 04:36:55]

Change committed as 285479