By setting a null current context, there is a brief window where all GL
calls will fail.  Since there is only one WGL context, just leave in on
the unreferenced window until a new window is made current.

BUG=angleproject:890

Change-Id: I51eadf23ca61e274f7d174ac5a9e0592bcdadebc
Reviewed-on: https://chromium-review.googlesource.com/265483Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>