This is Windows-only for now, but will be expanded to Linux. Also do
a bit of refactoring in our deqp.gypi file.

BUG=angleproject:1272

Change-Id: I526a99fe3b91a2d9e8901c7e033a4916a06780be
Reviewed-on: https://chromium-review.googlesource.com/320800Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>