Add missing override identifier for getSequence Fixed the compiling error in Mac by adding missing override identifier for getSequence. BUG=angleproject:1490 TEST=compile on macOS Sierra 10.12 Change-Id: I8f8d556c82885bbee1712e1b6e5f439b67906023 Reviewed-on: https://chromium-review.googlesource.com/396403 Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Commit-Queue: Corentin Wallez <cwallez@chromium.org>