Currently we use a globally unique symbol property on objects that get
passed from JavaScript to Go to store a unique ID that Go then uses when
referring back to the JavaScript object (via js.Value.ref). This
approach fails however when a JavaScript object cannot be modified, i.e.
cannot have new properties added or is frozen. The test that is added as
part of this commit currently fails with:

  Cannot add property Symbol(), object is not extensible

Instead we consolidate the string, symbol and object unique ID mapping
into a single map. Map key equality is determined via strict equality,
which is the semantic we want in this situation.

Change-Id: Ieb2b50fc36d3c30e148aa7a41557f3c59cd33766
Reviewed-on: https://go-review.googlesource.com/121799
Run-TryBot: Paul Jolly <paul@myitcv.org.uk>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>