[{eventType:"A",browsers:[{name:"Chrome",count:10}]},{eventType:"B",browsers:[{name:"Chrome",count:2}]},{eventType:"A",browsers:[{name:"Chrome",count:5},{name:"Safari",count:8}]}]

db.collection.aggregate([{"$unwind":"$browsers"},{"$group":{"_id":{"eventType":"$eventType","name":"$browsers.name"},"count":{"$sum":"$browsers.count"}}},{"$group":{"_id":"$_id.eventType","browsers":{"$push":{"name":"$_id.name","count":"$count"}}}},{$set:{eventType:"$_id"}},{$project:{"_id":0}},{"$out":"collection"}])