Remote object should be referenced by its members

[zcbenz]

This fixes another case that would cause the exception in #4733. In JS when you store a method of an object, the object will not be referenced, so in the following case calling stringify would throw exception because the remote.getGlobal('JSON') gets garbage collected.

let stringify = remote.getGlobal('JSON').stringify
stringify({})

In normal JS this is not a problem because the methods are objects themselves and rely on this pointer when being called. However in Electron the problem is the member methods of remote objects are not real functions, they just only records the object and the method's name, and send them to the main process when being called.

This PR works around it by referencing the remote object in its remote member methods, so as long as the methods are stored by user the remote object would never be garbage collected.

A real solution should be treating member methods as real functions, and get the remote object from the this pointer. But let's try it in another PR.

Close #4733.