vm: Copy missing properties from context

This addresses a current shortcoming of the V8 SetNamedPropertyHandler
function.

It does not provide a way to intercept Object.defineProperty(..) calls.
As a result, these properties are not copied onto the contextified
sandbox when a new global property is added via either a function
declaration or a Object.defineProperty(global, ...) call.

Note that any function declarations or Object.defineProperty() globals
that are created asynchronously (in a setTimeout, callback, etc.) will
happen AFTER the call to copy properties, and thus not be caught.

The way to properly fix this is to add some sort of a
Object::SetNamedDefinePropertyHandler() function that takes a callback,
which receives the property name and property descriptor as arguments.

Luckily, such situations are rare, and asynchronously-added globals
weren't supported by Node's VM module until 0.12 anyway.  But, this
should be fixed properly in V8, and this copy function should be removed
once there is a better way.

Fix #6416