[Tizen][Runtime] Enable pause/resume web application JS engine.

The Tizen core spec requires that the web runtime should have the ability to
pause and resume the renderer process and javascript engine when receiving 
the corresponding system events.

BUG=XWALK-1497,XWALK-1498
I think `is_suspended_` should only be changed after kill succeed. 

And some questions:
1. Where the handler of sigaction for `SIGTSTP`, I want to see how the program be suspended.
2. On today's meeting, there should a content api to call to stop the JS engine, could you sync with @shouqunliu?