Use the current default capture for the webaudio track to get the correct delay value

Use the current default capture for the webaudio audio track to get the correct delay value.

The source of the webaudio audio track might not have any capturer and this will make |need_audio_processing| flag to be false.

BUG=343866
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=251362

[no longer working on chromium, 2014-02-14 11:11:56]

Tommi, please review.

[tommi (sloooow) - chröme, 2014-02-14 11:55:07]

lgtm.  Please add an issue number if one exists.

[no longer working on chromium, 2014-02-14 12:05:57]

On 2014/02/14 11:55:07, tommi wrote:
> lgtm.  Please add an issue number if one exists.

Done, thanks.

[no longer working on chromium, 2014-02-14 12:06:03]

The CQ bit was checked by xians@chromium.org

[commit-bot: I haz the power, 2014-02-14 12:06:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/xians@chromium.org/166363002/80002

[commit-bot: I haz the power, 2014-02-14 14:23:34]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-02-14 14:23:35]

Retried try job too often on linux_chromeos for step(s) browser_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_chro...

[no longer working on chromium, 2014-02-14 16:09:53]

The CQ bit was checked by xians@chromium.org

[commit-bot: I haz the power, 2014-02-14 16:11:10]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/xians@chromium.org/166363002/80002

[commit-bot: I haz the power, 2014-02-14 17:24:27]

Change committed as 251362