Layout and input events messed up when using Flash in fullscreen

This happens because Flash content expects the content to be resized synchronously when going fullscreen, but that doesn't happen in webbrowser-app.

Chrome has a workaround for this in DesktopWindowTreeHostX11::SetFullscreen, with the following comment:

  // Try to guess the size we will have after the switch to/from fullscreen:
  // - (may) avoid transient states
  // - works around Flash content which expects to have the size updated
  // synchronously.
  // See https://crbug.com/361408
  if (fullscreen) {
    restored_bounds_in_pixels_ = bounds_in_pixels_;
    const gfx::Display display =
        gfx::Screen::GetScreenFor(NULL)->GetDisplayNearestWindow(window());
    bounds_in_pixels_ = ToPixelRect(display.bounds());
  } else {
    bounds_in_pixels_ = restored_bounds_in_pixels_;
  }

I guess we can do something similar in Oxide, where we resize the Flash content before the browser actually goes fullscreen