Determine the path to setup.exe via PathService rather than the parsed
command line. Only the former is authoritative.

BUG=141168

Review URL: https://codereview.chromium.org/169833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251720 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251720