This prevents chromium from showing up in git diff all the time after
applying patches.
It also prevents from accidentially updating the submodule sha sum.
A nice side effect is also that git diff is all of a sudden much
faster again. ;)

Change-Id: I6f48b8f90a2853b2e3716011c2c42bb661d09060
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>