Fuchsia: Implement sending unions from JS->C++

Bug: 883496
Change-Id: I2745399b530564c05e95f4375e04cac978454372
Reviewed-on: https://chromium-review.googlesource.com/c/1313417
Commit-Queue: Scott Graham <scottmg@chromium.org>
Reviewed-by: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605488}
4 files changed