WebView.fullscreen behaves differently for fullscreen Flash

When ordinary web content requests fullscreen, it's not granted until the application sets WebView.fullscreen to true. It doesn't have to do this synchronously inside the WebView.fullscreenRequested handler.

When Flash content requests fullscreen, the content will overlay the entire webview (which will effectively be in full screen mode) regardless of what the application does, and there's not much we can do about this.

What we should probably do is automatically cancel the fullscreen Flash if the application doesn't set WebView.fullscreen to true in the fullscreenRequested handler.