Fix offscreen browser browser settings nullptr

[illfang]

There is a bug with the optional browser settings in the offscreen browser project which will lead to a null poiinter exception if no settings are available.

Example:
https://github.com/illfang/CefSharp/tree/offscreen-browser-browser-settings-nullptr-test

System.NullReferenceException: Object reference not set to an instance of an object.
   at CefSharp.BrowserSettings.get_OffScreenTransparentBackground()
   at CefSharp.ManagedCefBrowserAdapter.CreateOffscreenBrowser(IntPtr windowHandle, BrowserSettings browserSettings, RequestContext requestContext, String address)
   at CefSharp.OffScreen.ChromiumWebBrowser.CreateBrowser(IntPtr windowHandle, String address, BrowserSettings browserSettings, RequestContext requestcontext)
   at CefSharp.OffScreen.ChromiumWebBrowser..ctor(String address, BrowserSettings browserSettings, RequestContext requestcontext, Boolean automaticallyCreateBrowser)

Probably related to dca52f9

I suggest two version to fix this. This is the first version which will always create the offscreen browser using the instance settings, request context and address. The instance browser settings are checked for null in the constructor.

The second solution is described in /pull/1359

I'm looking forward to your feedback. Feel free to close one of the PRs if you decide to merge one.

[amaitland]

I think this is the cleanest approach. The changes in dca52f9 were only from a week ago, so they've never been released.

[amaitland]

Will need to be back ported into the cefsharp/43 branch.

[amaitland]

Setting milestone to 43.0.1 as that's the first release this change will appear in (even though it's merged initially into master).