WinForms Expose Clearly defined place to call RegisterJsObject

Add isInitialized param to IWebBrowserInternal.OnInitialized()
Call IWebBrowserInternal.OnInitialized(false) before browser loaded and then IWebBrowserInternal.OnInitialized(true) after
For OffScreen and WPF only execute once (when isInitialized = true)
Update WinForms example to call RegisterJsObject