When retrieving BitSet masks return a const reference
of the mask to avoid invoking the copy-constructor.

Bug: angleproject:3877
Change-Id: I9f724a4864f3db17aa6f1ab74b81053c65313764
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2681767Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>