[Runtime Model] Implement non-persistent main document.

The following CL is to implement
https://crosswalk-project.org/jira/browse/XWALK-2.

The CL makes ApplicationProcessManager as an Observer for Runtime
objects. Therefore, whenever a Runtime object/Window is added or
removed, the ApplicationProcessManager could be notified.
Merge pull request #926 from shaochangbin/non-persistent-main-document

[Runtime] Implement Non-persistent main document