This generalizes GetOperatorString so that it can be used outside
ValidateLimitations.

BUG=angle:911

Change-Id: I43a160b3d7d60388ad529a72f1720d3171f67315
Reviewed-on: https://chromium-review.googlesource.com/249073Tested-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>