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/396403Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>