Includes angle::BitSetT changes from jmadill@chromium.org

BUG=angleproject:2169

Change-Id: I9f896613f5c6cdc91281cb9a00134f67291870d9
Reviewed-on: https://chromium-review.googlesource.com/804177Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>