Add url field in the detail of chrome.wallpaper.setWallpaper

Usage:
chrome.wallpaper.setWallpaper({ 'url' : 'http://your-domain.com/wallpaper.jpg',
                                'layout': 'CENTER_CROPPED' },
                              callback);
Note the origin of the provided url must be whitelisted in the app's manifest.
And only jpeg and png are supported.


BUG=176183

Review URL: https://codereview.chromium.org/26522002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227746 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227746