This CL fixes the new {CodeReference} class to comply to the style
guide. It makes it a proper class, renames private fields to end in an
underscore and simplifies the union declaration.

R=ahaas@chromium.org
CC=herhut@chromium.org

Bug: v8:7570
Change-Id: I329bbc6fca1ba3c0cb34fb4e1179eb4fa9044e76
Reviewed-on: https://chromium-review.googlesource.com/1023414Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52722}