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/261892Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Tested-by: Geoff Lang <geofflang@chromium.org>