Fix reference leak in Blit9. BUG=angleproject:1111 Change-Id: I0c71b765e8589a7387f4fd40f9dc453ed06c05b9 Reviewed-on: https://chromium-review.googlesource.com/289591 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>