Instead of accessing JsonValue struct fields directly, typed
accessors check that the tag matches with the type access.

Drive-by: The factory methods are now static methods on the JsonValue
type itself, making call-sites more readable.

R=tebbi@chromium.org

Bug: v8:8880
Change-Id: I49b37b3ba8eaf1153b8aa93ea08913077c923fdc
Reviewed-on: https://chromium-review.googlesource.com/c/1495559
Commit-Queue: Simon Zünd <szuend@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59968}