This CL fixes two problems with "stuck" wallpaper:

1) We should not wait infinitely for "wallpaper animation finished" event.
It will never happen if wallpaper is not loaded.

2) Wallpaper should always be reloaded in case of login screen "show" event.
It usually means we are returning from some dialog, which could use it's own
wallpaper.


BUG=322947, 320012

TEST=manual

Review URL: https://codereview.chromium.org/114393007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242501 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242501