Get system salt asynchronously from DeviceIDFetcher

Use GetSystemSalt instead of GetSystemSaltSync

BUG=141009, 306547

Review URL: https://codereview.chromium.org/34763002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230714 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230714