[ChromeOS] Clean up acounts page's use of chrome://userimage/
Use chrome://userimage/ for accounts pictures directly and get rid of
the user pictures cache that is no longer needed. This also resolves
issue chromium-os:15170 that is caused by racing condition between prefs
populate and user pictures cache update.
BUG=chromium-os:15170
TEST=Verify fix for chromium-os:15170.
Review URL: http://codereview.chromium.org/
7005035
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@85027 0039d316-1c4b-4281-b951-d872f2087c98