Fix a type error in WebChannel code. EventTarget is the native browser API, but WebChannelBaseTransport.Channel extends from goog.events.EventTarget, which is backed by the goog.events.Listenable API. Therefore, the goog.net.WebChannel interface should actually extend from goog.events.Listenable and not the native browser EventTarget class.
Fix a type error in WebChannel code. EventTarget is the native browser API, but WebChannelBaseTransport.Channel extends from goog.events.EventTarget, which is backed by the goog.events.Listenable API. Therefore, the goog.net.WebChannel interface should actually extend from goog.events.Listenable and not the native browser EventTarget class.

Without this change, teams using WebChannel could see incorrectly see errors:
ERROR - actual parameter 2 of EventTarget.addEventListener does not match formal parameter
found   : function(goog.net.WebChannel.ErrorEvent): undefined
required: (EventListener|function(Event): (boolean|undefined)|null)

TGP: https://test.corp.google.com/ui#id=OCL:205473783:BASE:205659331:1532378690496:1d627beb

RELNOTES: Fix type of the interface of goog.net.WebChannel to avoid compilation errors.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206080999

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