Poll X server for login prompt, in addition to watching file system

Again, I generally can't repro the race conditions that lead to our
tests failing to login sometime, but this change adds another layer of
checking to help avoid the race that sometimes leads to us attempting
to login when the prompt isn't there yet.

BUG=chrome-os-partner:711
TEST=BVT

Change-Id: Ia18294b70687bf79f6d826771fb16b80d6106bb6

Review URL: http://codereview.chromium.org/3342011
1 file changed