CL 60410 fixes the compiler such that reflect.StructField.PkgPath
is non-empty if and only if the field is unexported.
Given that property, we can cleanup the logic in the json encoder
to avoid parsing the field name to detect export properties.

Updates #21122

Change-Id: Ic01b9c4ca76386774846b742b0c1b9b948f53e7c
Reviewed-on: https://go-review.googlesource.com/65550
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>