Add missing override identifier for getSequence
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 <38ae6c0e2772a20e8e989f80cb8c99d274e44242@nvidia.com>
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>
Commit-Queue: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@chromium.org>

ecoal95/angle,ppy/angle,MSOpenTech/angle,ecoal95/angle,ecoal95/angle,ppy/angle,MSOpenTech/angle,ppy/angle,ppy/angle,MSOpenTech/angle,ecoal95/angle,MSOpenTech/angle,ecoal95/angle