Add isNaN to externs of chai.
Add isNaN to externs of chai.

https://www.chaijs.com/api/assert/#method_isnan

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=231303949

google/closure-compiler,google/closure-compiler,nawawi/closure-compiler,ChadKillingsworth/closure-compiler,Yannic/closure-compiler,monetate/closure-compiler,google/closure-compiler,monetate/closure-compiler,ChadKillingsworth/closure-compiler,ChadKillingsworth/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,Yannic/closure-compiler,nawawi/closure-compiler,Yannic/closure-compiler,nawawi/closure-compiler,nawawi/closure-compiler,Yannic/closure-compiler,google/closure-compiler,ChadKillingsworth/closure-compiler,vobruba-martin/closure-compiler,monetate/closure-compiler,vobruba-martin/closure-compiler,vobruba-martin/closure-compiler