This code doesn't actually call any D3D runtime methods, so it can be
included in our cross platform unit tests. Just include the varying
packing code for now.

BUG=angleproject:1296
BUG=angleproject:1638

Change-Id: I5c0ccccb00e1c13ca98448256be048f7cd0968ce
Reviewed-on: https://chromium-review.googlesource.com/417116
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>