Delay wallpaper load by 2 * average wallpaper load time.

BUG=279102

TEST=manual

Review URL: https://codereview.chromium.org/24625003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230138 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230138