dEQP support: use OSWindow instead of tcuWin32Window This is to help dEQP support become cross platform. BUG=angleproject:892 Change-Id: I4480b64a33b1725fb42769bf7dddef1b4ee27c83 Reviewed-on: https://chromium-review.googlesource.com/273594 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Corentin Wallez <cwallez@chromium.org>