Fix gl::ValidTextureTarget to work on all enum values. Change-Id: I347583ae1adcaaae9feeb1276f6150ceb8739885 Reviewed-on: https://chromium-review.googlesource.com/181557 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>