Fix IDBIndex.keyPath type
Fix IDBIndex.keyPath type

This may return any type which was specced in [W3C Last Call Working
Draft 16 May
2013](https://www.w3.org/TR/2013/WD-IndexedDB-20130516/#idl-def-IDBIndex).

This PR is prerequisite for google/closure-compiler#2035.

See
https://www.w3.org/TR/IndexedDB/#idl-def-IDBIndex
https://github.com/google/closure-compiler/pull/2035#issuecomment-286847194

Closes #811 from vobruba-martin/closure-library:indexeddb-keypath

RELNOTES: n/a

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152537558

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