Use asynchronous version GetSystemSalt from StartupAppLauncher

Use GetSystemSalt instead of GetSystemSaltSync.
GetSystemSalt is responsible to wait for service start up.

BUG=141009, 306547
R=satorux@chromium.org

Review URL: https://codereview.chromium.org/36233003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230352 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230352