Make OSWindow a non-static member of ANGLETest.

BUG=angle:611

Change-Id: I455e1dd0ad5582191621e316c4808a2d753e9aaa
Reviewed-on: https://chromium-review.googlesource.com/219867
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>