The xwalk::Runtime class (and related code) refactoring

The major changes (improvements):
- The Runtime class now only handles WebContents, it's unaware of the UI.
- The Runtime UI logic is encapsulated within the RuntimeUIDelegate implementation.
- Runtime life-cycle is definite - it is owned by the observer instead of being a suicide.
- Runtime::Close() causes the wrapped content::WebContents::Close() which had not happen(!) before.
- Multiple code clean-ups and simplification.