De-duplicate nodes in Node::ConvertNodeUnionsIntoNodes.
This makes calls to ParentNode.append() and similar methods that pass
the same node multiple times ignore all but the last occurrence of the
repeated node. This matches the behavior when a temporary
DocumentFragment was used.
Fixed: 375493447
Change-Id: I36035e09bfc2a78f53f2ef63a1506b08d28bb9b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5967474
Reviewed-by: Dominic Farolino dom@chromium.org
Commit-Queue: David Baron dbaron@chromium.org
Cr-Commit-Position: refs/heads/main@{#1375292}