[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
Seems it does not work, said ```error: request for member 'web_contents' in '* it.std::_Rb_tree_const_iterator<_Tp>::operator-><xwalk::Runtime*>()', which is of pointer type 'xwalk::Runtime* const' (maybe you meant to use '->' ?)```