WGPU: Flip y for the default framebuffer Bug: angleproject:389145696 Change-Id: I0d527ad3dc24dbca7e9d914b03edacdc257a568f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6477137 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Matthew Denton <mpdenton@chromium.org>