Adding UserManager checks so ProfileManager can be used in tests.

Changing GetDefaultProfile calls to Get*UserProfile means classes will access UserManager, which may not exist in tests of the old behavior.

BUG=322682
R=skuhne@chromium.org,rlp@chromium.org,dpolukhin@chromium.org

Review URL: https://codereview.chromium.org/109433011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240480 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240480