[Cleanup/iOS] Remove forward-declaration of non-existing class.
[Cleanup/iOS] Remove forward-declaration of non-existing class.

BUG=None

Review URL: https://codereview.chromium.org/1826993002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#383290}

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