Store iterator bitset by-value instead of by-reference. 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/293840 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>