Remove XKB cache files on the next boot after auto update.
Remove XKB cache files on the next boot after auto update.

The hook can be used for other tasks, that have to be
run after auto update, before X and Chrome start.

Note that impact on the boot time should be negligible as
checking "/mnt/stateful_partition/.install_completed" is
unlikely to touch the disk, as we already check existence
of files in the same directory (i.e. the file attributes
metadata is likely cached).

BUG=chromium-os:12916
TEST=confirm that /var/lib/xkb/*.xkm are removed as intended.

Change-Id: I07ced205fe44081609ebe3f30c79f350dce986c4

Review URL: http://codereview.chromium.org/6651018

joshfire/chromiumos-platform-init