Fix embedder comments about struct_size

[robert-ancell]

No description provided.

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

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[stuartmorgan-g]

test-exempt: comment-only change

(The bot should have known that, so the Cocoon settings for flutter/engine may be off?)

[cbracken]

Thanks! LGTM modulo suggested fix.