Add navigator.sendBeacon() use counters.

Source/modules/beacon/NavigatorBeacon.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://dvcs.w3.org/hg/webperf/raw-file/tip/specs/Beacon/Overview.html#Navigator
 
 [
     RuntimeEnabled=Beacon,
 ] partial interface Navigator {
     // FIXME: should be union type http://crbug.com/240176
-    [CallWith=ExecutionContext, RaisesException] boolean sendBeacon(DOMString url, ArrayBufferView data);
-    [CallWith=ExecutionContext, RaisesException] boolean sendBeacon(DOMString url, Blob data);
-    [CallWith=ExecutionContext, RaisesException] boolean sendBeacon(DOMString url, FormData data);
-    [CallWith=ExecutionContext, RaisesException] boolean sendBeacon(DOMString url, [Default=Undefined] optional DOMString data);
+    [CallWith=ExecutionContext, MeasureAs=SendBeacon, RaisesException] boolean sendBeacon(DOMString url, ArrayBufferView data);
+    [CallWith=ExecutionContext, MeasureAs=SendBeacon, RaisesException] boolean sendBeacon(DOMString url, Blob data);
+    [CallWith=ExecutionContext, MeasureAs=SendBeacon, RaisesException] boolean sendBeacon(DOMString url, FormData data);
+    [CallWith=ExecutionContext, MeasureAs=SendBeacon, RaisesException] boolean sendBeacon(DOMString url, [Default=Undefined] optional DOMString data);
 };