Don't populate the external view embedder in PaintContext. (#6686)
Don't populate the external view embedder in PaintContext. (#6686)

The platform views embedding is still WIP, and until we dynamically
merge the gpu and platform threads based on the presence of an embedded
view in the scene fetching the view embedder is not thread safe.

This PR essentially disables iOS platform views embedding, we will
re-enable once dynamic thread merging is supported.
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