[Application] Fix debug mode issues for extension process running in launcher

There are several DCHECK issues:
1. DBus FileDescriptor validity check should runs on IO allowed thread;
2. AtExitManager should be created for registering dtor callbacks;
3. XWalkExtensionProcess should be destroyed on its own thread.
Merge pull request #1895 from seanlong/ep_launcher_fix

[Application] Fix debug mode issues for extension process running in launcher