src: don't overwrite non-writable vm globals

Check that the property doesn't have the read-only flag set before
overwriting it.

This is Ben Noordhuis previous commit, but keeping
is_contextual_store. is_contextual_store describes whether
this.foo = 42 or foo = 42 was called. The second is contextual
and will fail in strict mode if foo is used without
declaration. Therefore only do an early return if it is
a contextual store. In particular, don't do an early
return for Object.defineProperty(this, ...).

Fixes: https://github.com/nodejs/node/issues/10223
Refs: https://github.com/nodejs/node/pull/10227
PR-URL: https://github.com/nodejs/node/pull/11109


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>