This commit changes the encoding of js.Value so that the zero js.Value
represents the JavaScript value "undefined". This is what users
intuitively expect.

Specifically, the encodings of "undefined" and the number zero have
been swapped.

Fixes #27592.

Change-Id: Icfc832c8cdf7a8a78bd69d20e00a04dbed0ccd10
Reviewed-on: https://go-review.googlesource.com/c/143137
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>