Implementations of chrome.system.* for ios (cpu, display, memory, network)

- Add auto tests for chrome.system.cpu plugin
- Initial stub implementation of chrome.system.cpu.getInfo for ios
- Add auto tests for chrome.system.display plugin
- Implementation of chrome.system.display.getInfo for ios
nit: Obj-C allows you to define variables not at the top of a block. Seeing uninitialized variables makes the code a bit harder to read (at least for me), so I'd just define them when assigned.

Since len is an in-out, should initialize it to 0 I think.