change owner of /var/lib/xkb to user xorg

As the X server now runs using user xorg credentials instead of root
/var/lib/xkb needs to be similarly owned by user xorg so X server
can write keymaps there when switching VTs. Right now it falls
back to writing them to /tmp.

BUG=chromium:331553
TEST=xkm files should be written to /var/lib/xkb instead of /tmp
TEST=verify you can switch to VT2 and back
TEST=run platform_FilePerms test

Change-Id: I858ef73e4d5a10daf0eaed8bc20a87a518a8ee35
Signed-off-by: Dominik Behr <dbehr@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/184419
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
diff --git a/ui.conf b/ui.conf
index 5ffc271..3f83356 100644
--- a/ui.conf
+++ b/ui.conf
@@ -45,6 +45,7 @@
   # XKB writes keymaps here; otherwise things like Ctrl-Alt-Fx VT switching
   # don't work.
   mkdir -p /var/lib/xkb
+  chown xorg:xorg /var/lib/xkb
 
   # Make sure we we can easily track UI state.
   rm -rf /var/run/state