Move getData from the D3D to the Impl. We will need this functionality for index validation, so we likely can assume an implemenation will need to support such a call. We might redesign this requirement, but for now we can't call though to BufferD3D on OSX. BUG=angle:773 Change-Id: I16187c78ff62471fdf7221a11b9956560e617a14 Reviewed-on: https://chromium-review.googlesource.com/238450 Tested-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>