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Previously we assumed nullable unions were always encoded as/decoded
from pointers, but unions are only encoded as/decoded from pointers
when they are nested in another union.

This CL changes encodeUnion/decodeUnion to use encodeUnionAsPointer()
and decodeUnionFromPointer() when a union field is another union.

Fixed: 1101849
Change-Id: I7615207fe4052eadb544749fe89225c37d52fa58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2325458Reviewed-by: Dominick Ng <dominickn@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#794363}