Add wallpaper api tests

This CL add the following tests:
1. test chrome.setWallpaper can set ChromeOS wallpaper from jpg or png formatted arraybuffer.
2. test chrome.setWallpaper can set ChromeOS wallpaper from a valid url that point to a jpg or png image.
3. test an invalide URL wont set wallpaper
4. test a new wallpaper request will cancel previous unfinished request


BUG=176183

Review URL: https://codereview.chromium.org/153093010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251087 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251087