Tighten up nullability on the Closure HtmlSanitizer interface.
Tighten up nullability on the Closure HtmlSanitizer interface.

RELNOTES: N/A

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170718232

platinumpixs/google-closure-library,google/closure-library,lucidsoftware/closure-library,teppeis/closure-library,google/closure-library,lucidsoftware/closure-library,google/closure-library,platinumpixs/google-closure-library,google/closure-library,platinumpixs/google-closure-library,teppeis/closure-library,lucidsoftware/closure-library,teppeis/closure-library,google/closure-library,platinumpixs/google-closure-library,lucidsoftware/closure-library,teppeis/closure-library