Clean up and simplify UserImageLoader
This CL simplifies the UserImageLoader. A foreground_task_runner_ and a
background_task_runner_ are set during construction, providing well-
defined task runners for all tasks. There no longer is a need for locking
as all callbacks from the ImageDecoders arrive via the same task runner.
The CL also adds a method that processes image data already present in
memory, in preparation for policy-set avatars and wallpapers.
BUG=152957,152959,220418
TEST=Manual
Review URL: https://codereview.chromium.org/
69293002
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235136 0039d316-1c4b-4281-b951-d872f2087c98