chromeos: Wait for service to be available when getting system salt

Add CryptohomeClient::WaitForServiceToBeAvailable().
Use WaitForServiceToBeAvailable() from SystemSaltGetter.

BUG=141009
R=satorux@chromium.org

Review URL: https://codereview.chromium.org/34303002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230067 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230067