Presentation API: add UMA.

Source/modules/presentation/PresentationRequest.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.
 
 // TODO(mlamouri): double-check link after spec changes have landed.
 // https://w3c.github.io/presentation-api/#interface-presentationrequest
 
 [
     Constructor(DOMString url),
     ConstructorCallWith=ExecutionContext,
     RaisesException=Constructor,
     GarbageCollected,
     RuntimeEnabled=Presentation,
+    MeasureAs=PresentationRequestConstructor
 ] interface PresentationRequest : EventTarget {
-    [CallWith=ScriptState] Promise<PresentationSession> start();
-    [CallWith=ScriptState] Promise<PresentationSession> join(DOMString id);
-    [CallWith=ScriptState] Promise<PresentationAvailability> getAvailability();
+    [CallWith=ScriptState, MeasureAs=PresentationRequestStart] Promise<PresentationSession> start();
+    [CallWith=ScriptState, MeasureAs=PresentationRequestJoin] Promise<PresentationSession> join(DOMString id);
+    [CallWith=ScriptState, MeasureAs=PresentationRequestGetAvailability] Promise<PresentationAvailability> getAvailability();
 
     attribute EventHandler onsessionconnect;
 };