GroupElement.php: Use strict mode in array_search
GroupElement.php: Use strict mode in array_search

HHVM's fix for https://bugs.php.net/bug.php?id=62171 
(https://github.com/facebook/hhvm/issues/1349) is broken
in a way we haven't yet perfectly isolated, but which we
can work around by invoking array_search() in strict mode.
We should be doing that anyway.

Bug: T131509
Change-Id: I37d003c55703c1f5aca38ae1370447c1c1d176fd

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui