This prevents a race between starting Xvfb on test bots
and X11 calls in X11Window::setVisible(),
which used to cause flaky hangs on Linux SwANGLE bots.

Unfortunately, in order to hide SwiftShader OS window,
it must be a separate window from other backends,
so it is no longer possible to have a single window for all backends,
even if we don't reuse EGL Display.

The only platform that still uses a single OS Window is Android,
since there is only one system window per test application.

In addition, all the tests that make OS Window visible explicitly,
no longer do this for SwiftShader device.

Bug: angleproject:4434
Change-Id: I1a067c22bfeee9288046b9d9566740731c0d627c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2125945
Commit-Queue: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>