Add the beforeinput event type.
Add the beforeinput event type.

See https://w3c.github.io/input-events/

RELNOTES:goog.events.EventType is updated with beforeinput event.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148476380

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