Video Player: fix misc bugs

Video Player: fix misc bugs

- Add null-checks 
- Return an empty mime/thumbnailUrl instead of error if request is succeeded (media_manager.js)
- Make savedVolume_ consistent (media_controls.js)

BUG=none
TEST=manual

Committed: https://crrev.com/f1305fc02d43f2ad611257b7d787a66b29507deb
Cr-Commit-Position: refs/heads/master@{#293328}

[yoshiki, 2014-09-04 08:09:54]

yoshiki@chromium.org changed reviewers:
+ fukino@chromium.org

[yoshiki, 2014-09-04 08:10:06]

@fukino-san, PTAL. Thanks.

[fukino, 2014-09-04 08:35:26]

On 2014/09/04 08:10:06, yoshiki wrote:
> @fukino-san, PTAL. Thanks.

lgtm.

[yoshiki, 2014-09-04 11:20:40]

The CQ bit was checked by yoshiki@chromium.org

[commit-bot: I haz the power, 2014-09-04 11:21:54]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/yoshiki@chromium.org/537863002/40001

[commit-bot: I haz the power, 2014-09-04 20:09:29]

Committed patchset #3 (id:40001) as 235a50fdb2b79a6f6e0970a655f58430ee341dce

[commit-bot: I haz the power, 2014-09-10 03:32:24]

Patchset 3 (id:??) landed as
https://crrev.com/f1305fc02d43f2ad611257b7d787a66b29507deb
Cr-Commit-Position: refs/heads/master@{#293328}