WinForms: don't report size changed when Size is 0x0.

This occurs when the control is on a modeless Form and it is minimized.  Similar guard as in my PR is in cefclient.

To reproduce the problem, replace BrowserFormLoad() in CefSharp.WinForms.Example\BrowserForm.cs with the following...

        private void BrowserFormLoad(object sender, EventArgs e)
        {
            ToggleBottomToolStrip();

            Form f = new Form();
            f.Controls.Add(new ChromiumWebBrowser(CefExample.DefaultUrl) {
                Dock = DockStyle.Fill,
            });
            f.Show();
        }

Then,
1. start the example app,
2. minimize the new modeless dialog,
3. then restore it.
4. Observe that Chrome has not been repainted, and the form's grey background is showing.