Presentation API: add UMA.

Source/modules/presentation/Presentation.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-presentation
 
 [
     GarbageCollected,
     RuntimeEnabled=Presentation
 ] interface Presentation : EventTarget {
-    attribute PresentationRequest? defaultRequest;
+    [MeasureAs=PresentationDefaultRequest] attribute PresentationRequest? defaultRequest;
 
     // This API used by presenting browsing context.
     // TODO(avayvod): The spec has |getSession()| and |getSessions()| instead.
     readonly attribute PresentationSession? session;
     // TODO(avayvod): attribute EventHandler onsessionavailable;
 };