Determine guest mode status for metrics collection

Modification to the function IsGuestMode. It returns true only if the guestfs
partition is mounted and the file /var/run/state/logged-in exits. Previously
it did not check for this file existence.

BUG=chromium-os:15763
TEST=Log in in to guest mode right after installing recovery image and check
the absence of /home/chronos/prev_stats. Log out and log in as a user and
confirm the existence of this directory.

Change-Id: I29c66642ba590e882e6ec7a02732e818c87ac2ad
Reviewed-on: http://gerrit.chromium.org/gerrit/1586
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Arkaitz Ruiz Alvarez <arkaitzr@google.com>
diff --git a/metrics/metrics_library.cc b/metrics/metrics_library.cc
index 8c98696..0f2a7d9 100644
--- a/metrics/metrics_library.cc
+++ b/metrics/metrics_library.cc
@@ -126,7 +126,7 @@
                        &result)) {
     return false;
   }
-  return result;
+  return result && access("/var/run/state/logged-in", F_OK) == 0;
 }
 
 bool MetricsLibrary::AreMetricsEnabled() {