Fuchsia: Implement receive of unions to JS from C++

Bug: 883496
Change-Id: I402ef87f893488bcd15ec171e18add9918022a95
Reviewed-on: https://chromium-review.googlesource.com/c/1313570
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605528}
3 files changed