[chrome.displaySource] Session notification improvements

extensions/common/api/display_source.idl

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // The <code>chrome.displaySource</code> API creates a Display
 // session using WebMediaStreamTrack as sources.
 namespace displaySource {
   enum ErrorType {
     // Cannot create media pipeline from the given media stream which could be
     // appropriate for a Display session (e.g., necessary codecs are missing
@@ skipping 126 matching lines @@
     // |callback| : Called when the session is terminated.
     [nocompile] static void terminateSession(
         long sinkId, optional CallCompleteCallback callback);
   };
 
   interface Events {
     // Event fired when the available sinks are modified (either their amount
     // or properties)
     // |sinks| the list of all currently available sinks
     static void onSinksUpdated(SinkInfo[] sinks);
-    // Event fired when the Display session is started.
-    // |sinkId| Id of the peer sink
-    [nocompile] static void onSessionStarted(long sinkId);
     // Event fired when the Display session is terminated.
     // |sinkId| Id of the peer sink
     [nocompile] static void onSessionTerminated(long sinkId);
     // Event fired when an error occurs.
     // |sinkId| Id of the peer sink
     // |errorInfo| error description
     [nocompile] static void onSessionErrorOccured(long sinkId,
                                                   ErrorInfo errorInfo);
   };
 };