Use dirty bit for element array buffer BUG=angleproject:2188 Change-Id: I2b2aced542032c7c263f911ef1516af1d42190cc Reviewed-on: https://chromium-review.googlesource.com/846346 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>