Add a Buffer::getIndexRange function. Instead of exposing the index range cache, add a more generic method that allows the buffers to handle their own caching of index ranges. BufferImpl::getData can be hard to implement for BufferGL because there isn't a way to tell the buffer to unmap and glGetBufferSubData can be very expensive, requiring an extra copy of the data. BUG=angleproject:881 Change-Id: Idec645219056132e0d72a410fbe7b971fa02c9e9 Reviewed-on: https://chromium-review.googlesource.com/261892 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Olli Etuaho <oetuaho@nvidia.com> Tested-by: Geoff Lang <geofflang@chromium.org>