Presentation API: implement .getAvalability() (Blink side).

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 00fee6e5b0a33f7e969390880b1b12c43901c8df..0c104b5a76e1b343fea06b526446256c988920b0 100644
--- a/Source/modules/presentation/Presentation.idl
+++ b/Source/modules/presentation/Presentation.idl
@@ -13,6 +13,7 @@
     [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;
     attribute EventHandler ondefaultsessionstart;
+
+    [CallWith=ScriptState] Promise<PresentationAvailability> getAvailability(DOMString presentationUrl);
 };