Allow the first created WebContext to become the default context in single process mode

[Chris Coulson]

In single process mode, webviews always use the default WebContext (and WebView.context is basically read-only). When running an application with OXIDE_SINGLE_PROCESS in the environment, we allow an application provided WebContext to become the default WebContext, so that you can run most existing applications (assuming they only ever create a single context) with OXIDE_SINGLE_PROCESS and they won't break.

When setting single process via the API, we don't allow any application provided WebContext to become the default. This is making it difficult to run tests though, where we do want to create a TestWebContext and have it used by webviews during the tests.