Refactoring of the UserImageManager.

BUG=313303,322682
TEST=browser_tests:UserImageManager*

Review URL: https://codereview.chromium.org/106093008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242448 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242448