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This CL did the following:

1. Save/update manifest in chrome.storage.local when user successfully
requested latest manifest from server. If user failed to get manifest
from server(offline or server error), fallback to the manifest saved
in chrome.storage.local last time.

2. Lazily saves all requested thumbnails to thumbnails directory. And
load from that directory when user open wallpaper picker next time. Note
that thumbnails are shared across user session. So after one user saved
thumbnails, all other users will directly use the thumbnails that saved
in thumbnail directory.


BUG=158668


Review URL: https://chromiumcodereview.appspot.com/11348215

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@171011 0039d316-1c4b-4281-b951-d872f2087c98