Move the index range cache to gl::Buffer. Because we want to move all validation to the API layer, we need to move the index validation to the API layer. This means exposing the index cache to the validation layer. In the future we will probably want a way to skip index validation when it's not necessary, or the chosen back-end handles index validation on its own. BUG=angle:571 Change-Id: Iee1618e669bc28abf7e0709ca4a03b4727b1383e Reviewed-on: https://chromium-review.googlesource.com/210646 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>