Implement robust GetShaderiv entry points. BUG=angleproject:1354 Change-Id: I994b7a596b5b67fadbd91ad0d64054c792523200 Reviewed-on: https://chromium-review.googlesource.com/395111 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>