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.