Implement import of fuchsia external objects passed by zircon handle. This
works exactly the same as with file descriptors.

Bug: angleproject:3492
Change-Id: I4d46917dfc5902f00c94550158a9f8073097f0a4
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1642334
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>