The by-reference method was giving strange problems when using a non-
LValue as argument to IterateBitSet.

BUG=angleproject:1040

Change-Id: Iec10f7256a5b19b239804e0beba4ae675f53f876
Reviewed-on: https://chromium-review.googlesource.com/293840Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>