[cryptohome] make guest-mode homedir mount with -rwx------

BUG=chromium-os:13173
TEST=unit tests, manual: ls -ald /home/chronos/user -> drwx------ 10 chronos chronos 500 Apr 21 14:26 /home/chronos/user/

Change-Id: Ifd60b6c31e773d55d334a6f6c58565c2c64365b8

R=jimhebert@chromium.org

Review URL: http://codereview.chromium.org/6895001
diff --git a/mount.cc b/mount.cc
index ff2106f..6b6f471 100644
--- a/mount.cc
+++ b/mount.cc
@@ -835,7 +835,7 @@
   current_user_->Reset();
 
   // Attempt to mount guestfs
-  if (!platform_->Mount("guestfs", home_dir_, "tmpfs", "")) {
+  if (!platform_->Mount("guestfs", home_dir_, "tmpfs", "mode=0700")) {
     LOG(ERROR) << "Cryptohome mount failed: " << errno << " for guestfs";
     return false;
   }