We only ever call this method inside the D3D Renderer, so we can
downcast to BufferD3D and call getData on the D3D-specific type.

Leave a FIXME for handling index range validation, which will
need a CPU-side data cache.

Change-Id: Iaf71bc8055869a8561777b6b36f67e376a1d0b81
Reviewed-on: https://chromium-review.googlesource.com/224654Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>