Goal:

Properly contain Swarming tasks. There is two goals:
- Enable enforcing limits, like number of processes.
- Detect the whole process group lifetime.

We are particularly interested in the later, especially as part of making issue 732811 much better. Issue 732811 would likely still be needed (e.g. a complete host process list) as on Windows, it is still fairly trivial to escape the job object.

Nested job objects are only supported on Windows 8 and later yet many Swarming tasks actively use job objects internally.
Ref: https://msdn.microsoft.com/en-us/library/windows/desktop/hh448388.aspx

So there are two options on Windows 7:
- Only set the job object when requested at task recreation, for the subset of tasks which do not use job object as part of the task
- Only use the job object on Win8 and later

I'm learning towards the later as the former represents a lot of management that will likely never be done properly.