Fix low risk non-nullable => nullable attributes / arguments to match the spec

third_party/WebKit/Source/modules/presentation/PresentationConnection.idl

 // Copyright 2014 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.
 
 // https://w3c.github.io/presentation-api/#interface-presentationconnection
 
 enum PresentationConnectionState {
     "connected",
     "closed",
     "terminated"
 };
 
 [
     RuntimeEnabled=Presentation
 ] interface PresentationConnection : EventTarget {
-    readonly attribute DOMString? id;
+    readonly attribute DOMString id;

[foolip, 2016/12/14 19:17:12]

Can PresentationConnection::m_id ever be a null WTF::String? Spec-side in
https://w3c.github.io/presentation-api/#dfn-presentation-identifier it seems
like it never could be, but better make sure.

[lunalu1, 2016/12/15 11:33:59]

Will ask mlamouri@ to confirm that.

     readonly attribute PresentationConnectionState state;
     [MeasureAs=PresentationConnectionClose] void close();
     [MeasureAs=PresentationConnectionTerminate] void terminate();
     attribute EventHandler onconnect;
     attribute EventHandler onclose;
     attribute EventHandler onterminate;
 
     // Communication
     attribute BinaryType binaryType;
     attribute EventHandler onmessage;
     [RaisesException, MeasureAs=PresentationConnectionSend] void send(DOMString message);
     [RaisesException, MeasureAs=PresentationConnectionSend] void send(Blob data);
     [RaisesException, MeasureAs=PresentationConnectionSend] void send(ArrayBuffer data);
     [RaisesException, MeasureAs=PresentationConnectionSend] void send(ArrayBufferView data);
 };