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
Using "get" prefix in Obj-C doesn't follow their style-guides. Probably fine in our code (not worth changing), but FYI in Obj-C this should be called:

rotationForDisplay: