fix #553 by handling Initial resize and OnSizeChanged

This fixes two things:

1st, After re-sizing sometimes the browser does not react correctly. Changed from handling/forcing `OnPaint` to handling  `OnSizeChanged`.

2st, #553 : The line added to `CefSharp.WinForms.Example\BrowserForm.cs` causes the bug. The call to `Resize()` inside `IWebBrowserInternal.OnInitialized` fixes it.
I believe in a few rare cases that `OnSizeChanged` was being called even whilst disposal was happening, so we'll likely  still need the `managedCefBrowserAdapter != null` check.