Implemented fake UpdateEngineClient for AU manual tests.

BUG=270297
TEST=none

Review URL: https://codereview.chromium.org/32323003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230179 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230179