Add using-declaration to support Googletest roll. Without this change, rolling Googletest results in compilation errors: https://crrev.com/c/1594628/2. Change-Id: I85e122ad449a3d8fb155b07f64d69022294ab6cd Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1598309 Reviewed-by: Victor Costan <pwnall@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Victor Costan <pwnall@chromium.org>