Can't use multiple contexts in single process mode

[Chris Coulson]

On the browser side, RenderProcessHost is associated with a single BrowserContext instance, and the render process only displays views from this context. In single-process mode there is only one RenderProcessHost associated with the first BrowserContext, so it's not really possible to use more than one context properly.

This means that in single process mode, some features and settings of additional contexts will be ignored - these include:
- User agent string
- User scripts
- WebContextDelegateWorkers
- Cache and data directories