* Use weak pointer to avoid race condition
* Use DeleteSoon to delete pointer across threads
* Simplify EventSubscriber
* No need to manually mange V8 convertions
* Fix cpplint warning
We should update cpplint for this, but let's do it in other PR.
* Move UI thread operations to EventSubscriber
* Less and more assertions
Some methods are now private so no more need to assert threads.
* Fix cpplint warnings
* No longer needs the EventEmitted
* EventSubscriber => StreamSubscriber
* Reduce the copies when passing data
* Fix cpplint warnings
Some callbacks are stored in native resources that not managed by
JavaScript, so when those resources are destroyed the JavaScript context
may already be destroyed, and releasing callbacks then would result in
crash.
