Adding UMA metrics to track usage of pointerevents.

third_party/WebKit/Source/core/events/PointerEvent.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.
 
 // https://w3c.github.io/pointerevents/#pointerevent-interface
 
 [
     Constructor(DOMString type, optional PointerEventInit eventInitDict),
     RuntimeEnabled=PointerEvent,
 ] interface PointerEvent : MouseEvent {
-    readonly    attribute long      pointerId;
-    readonly    attribute double    width;
-    readonly    attribute double    height;
-    readonly    attribute float     pressure;
-    readonly    attribute long      tiltX;
-    readonly    attribute long      tiltY;
-    readonly    attribute DOMString pointerType;
-    readonly    attribute boolean   isPrimary;
+    [MeasureAs=PointerEventCount]   readonly    attribute long      pointerId;

[mustaq, 2016/04/19 19:11:43]

Since this is counting attribute accesses, I would make the name more focused.
What about "PointerEventAttributeCount" instead?

+    [MeasureAs=PointerEventCount]   readonly    attribute double    width;
+    [MeasureAs=PointerEventCount]   readonly    attribute double    height;
+    [MeasureAs=PointerEventCount]   readonly    attribute float     pressure;
+    [MeasureAs=PointerEventCount]   readonly    attribute long      tiltX;
+    [MeasureAs=PointerEventCount]   readonly    attribute long      tiltY;
+    [MeasureAs=PointerEventCount]   readonly    attribute DOMString pointerType;
+    [MeasureAs=PointerEventCount]   readonly    attribute boolean   isPrimary;
 };