[ios] Make sure web::EvaluateJavaScript returns for nil result.
[ios] Make sure web::EvaluateJavaScript returns for nil result.

WKWebView provides nil for JS undefined value.

BUG=571564

Review URL: https://codereview.chromium.org/1582303003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#369518}

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