Added plumbing for the availablechange event from Blink to WebPresentationClient.

Source/modules/presentation/Presentation.idl

Index: Source/modules/presentation/Presentation.idl
diff --git a/Source/modules/presentation/Presentation.idl b/Source/modules/presentation/Presentation.idl
index bd39e1d02c0ea7cfd994700e45c6d63409f11996..92b53bbf4205109c47a6d6b5355e3496b7ecd338 100644
--- a/Source/modules/presentation/Presentation.idl
+++ b/Source/modules/presentation/Presentation.idl
@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// http://webscreens.github.io/presentation-api/#navigatorpresentation
+// https://w3c.github.io/presentation-api/#navigatorpresentation
 
 [
     GarbageCollected,
@@ -10,8 +10,8 @@
 ] interface Presentation : EventTarget {
     readonly attribute PresentationSession? session;
 
-    [CallWith=ScriptState] Promise<PresentationSession> startSession(DOMString senderId, [Default=Undefined] optional DOMString presentationId);
-    [CallWith=ScriptState] Promise<PresentationSession> joinSession(DOMString senderId, [Default=Undefined] optional DOMString presentationId);
+    [CallWith=ScriptState] Promise<PresentationSession> startSession(DOMString presentationUrl, [Default=Undefined] optional DOMString presentationId);
+    [CallWith=ScriptState] Promise<PresentationSession> joinSession(DOMString presentationUrl, [Default=Undefined] optional DOMString presentationId);
 
     attribute EventHandler onavailablechange;
 };