Fix searching for DirectX SDK during GN build.

Before that GN just checked for DXSDK_DIR environment variable.
GYP does more and checks registry, let's do the same in GN.

R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37599004

Patch from Vyacheslav Chigrin <vchigrin@yandex-team.ru>.

git-svn-id: http://webrtc.googlecode.com/svn/trunk@8066 4adac7df-926f-26a2-2b94-8c16560cd09d