[ios] Corrected Closure annotation for getEmbedChild_ function.
[ios] Corrected Closure annotation for getEmbedChild_ function.

Old annotation was HTMLElement, new one is Node.

BUG=593002

Review URL: https://codereview.chromium.org/1779453002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#379854}

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