This patch adds the missing return value *this in the operator=
of class BitSetT

BUG=angleproject:1814

Change-Id: I758d62e80e4ade92aac64315f1808ca1eabdc787
Reviewed-on: https://chromium-review.googlesource.com/487423
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>