Move operator definitions to their own file This generalizes GetOperatorString so that it can be used outside ValidateLimitations. BUG=angle:911 Change-Id: I43a160b3d7d60388ad529a72f1720d3171f67315 Reviewed-on: https://chromium-review.googlesource.com/249073 Tested-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Zhenyao Mo <zmo@chromium.org>