Usually we copy data from the Image classes into the Storage. This
method, which will only apply to D3D11, is useful for our SubData
performance workaround.

BUG=angle:729

Change-Id: I50ca8158ee9e0638f59f617b9e6170b508eaeb44
Reviewed-on: https://chromium-review.googlesource.com/219837Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>