[Wallpaper Picker] Fixed custom wallpaper Context Menu z-index

Fixed custom wallpaper Context Menu z-index

As you can see in `src/ui/webui/resources/css/menu.css`, the z-index CSS property for the menu is 3.
Since the z-index CSS property of the check Image is also 3, it causes some UI glitches like the one below.

I've set it to 2 since 1 is used by thumbnails.

OLD=http://i.imgur.com/HfCtRvJ.png
NOW=http://i.imgur.com/nXkLa3b.png

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=255916

[François Beaufort, 2013-12-27 14:51:51]

Hello Biao,

Just found a small UI issue in the Wallpaper Context Menu.

Happy New Year!

[bshe, 2014-01-10 15:31:48]

On 2013/12/27 14:51:51, François Beaufort wrote:
> Hello Biao,
> 
> Just found a small UI issue in the Wallpaper Context Menu.
> 
> Happy New Year!

lgtm. great! Thanks for fixing it.

[François Beaufort, 2014-03-08 08:55:03]

The CQ bit was checked by beaufort.francois@gmail.com

[commit-bot: I haz the power, 2014-03-08 11:13:28]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/beaufort.francois@gmail.com/121723002/1

[commit-bot: I haz the power, 2014-03-08 13:25:34]

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

[commit-bot: I haz the power, 2014-03-08 13:25:34]

Retried try job too often on win_rel for step(s) unit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=win_rel&nu...

[François Beaufort, 2014-03-10 07:45:14]

The CQ bit was checked by beaufort.francois@gmail.com

[commit-bot: I haz the power, 2014-03-10 07:45:20]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/beaufort.francois@gmail.com/121723002/1

[commit-bot: I haz the power, 2014-03-10 12:11:10]

Change committed as 255916