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
SVN changes up to revision 235136
Update V8 to version 3.23.4.

TBR=jkummerow@chromium.org
BUG=

Review URL: https://codereview.chromium.org/68203024

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@235137 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 235137