If duration of media source is unknown then return NaN.

If duration of media source is unknown then return NaN.

According to spec http://www.w3schools.com/tags/av_prop_duration.asp,
if no audio/video is set, NaN should be returned. When metadata is not
available, duration is unknown then NaN is returned.

R=qinmin@chromium.org

Committed: https://crrev.com/536be704e6ff71001c7f402e18956204bf76f8bf
Cr-Commit-Position: refs/heads/master@{#294131}

[qinmin, 2014-09-09 21:54:32]

lgtm

[atsykhonia, 2014-09-10 05:46:47]

The CQ bit was checked by atsykhonia@yandex-team.ru

[commit-bot: I haz the power, 2014-09-10 05:49:30]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/atsykhonia@yandex-team.ru/552723003/1

[commit-bot: I haz the power, 2014-09-10 06:58:36]

Committed patchset #1 (id:1) as 082d298e9c7150a5a05bb4a7d3f45604f050409d

[commit-bot: I haz the power, 2014-09-10 07:03:14]

Patchset 1 (id:??) landed as
https://crrev.com/536be704e6ff71001c7f402e18956204bf76f8bf
Cr-Commit-Position: refs/heads/master@{#294131}