[login_manager] Fix check for existence of SERIAL_MARKER
[login_manager] Fix check for existence of SERIAL_MARKER

We were trying to run a command called SERIAL_MARKER, instead of checking
for the existence of a file called  due to a mixup
in punctuation :-)

BUG=None
TEST=run code, ensure that no error is at the top of /var/log/ui/ui.LATEST

Change-Id: I85ef2d346ad4337426f2c1dae8eb8368a8ac3e0e
Reviewed-on: http://gerrit.chromium.org/gerrit/7556
Tested-by: Chris Masone <98e3d65c6b0e9e2391eb4c2e8b1e572b77e544ed@chromium.org>
Reviewed-by: Scott James Remnant <6b88cbe64f92c5d64ffbf57d7b3d2efc0467fe34@chromium.org>
Reviewed-by: Terry Lambert <f31fd7e0e83d662934f80cc9a2fd8f7385851082@chromium.org>

chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager,chadversary/chromiumos.platform.login_manager