Mount external drives as synchronous (write-through mode) by default.

BUG=chromium-os:16113
TEST=Inserted USB and SD drives to a Cr48 and checked that sync was in the mount flags.

Change-Id: I37b61ddf33a7c4b92ca579f05215f48296d62b67
Reviewed-on: http://gerrit.chromium.org/gerrit/2101
Reviewed-by: Puneet Kumar <puneetster@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
diff --git a/chromeos_mount.cc b/chromeos_mount.cc
index dc4ec4e..2b7e784 100644
--- a/chromeos_mount.cc
+++ b/chromeos_mount.cc
@@ -31,6 +31,7 @@
     "nodev",
     "noexec",
     "nosuid",
+    "sync",
     NULL
 };
 const char* kDefaultUnmountOptions[] = {