Support different Chromium process models

Chromium does support four different process models:

• Process-per-site-instance
• Process-per-site
• Process-per-tab
• Single process

We only officially support Process-per-site-instance, which is the most secure, but also the one with the most overhead. "Single Process" is only available for debugging and development purposes. The other modes might be beneficial especially for generic browsers with lots of tabs on memory constrained devices.

In any case, we should document what we support, and how.