Refactored Texture2DArray, separated out D3D logic BUG=angle:688 Change-Id: Ie3e5ac2bedcffb816ef1537575f09afb3f5e7a35 Reviewed-on: https://chromium-review.googlesource.com/208381 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Brandon Jones <bajones@chromium.org>