Update closure event types to include Media Source Extensions events.
Update closure event types to include Media Source Extensions events.

MediaSource events:
https://www.w3.org/TR/media-source/#mediasource-events

SourceBuffer events:
https://www.w3.org/TR/media-source/#sourcebuffer-events

RELNOTES:goog.events.EventType is updated with Media Source Extensions events.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=149952469

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