Adding UMA metrics to track usage of pointerevents.

third_party/WebKit/Source/core/events/PointerEvent.idl

Index: third_party/WebKit/Source/core/events/PointerEvent.idl
diff --git a/third_party/WebKit/Source/core/events/PointerEvent.idl b/third_party/WebKit/Source/core/events/PointerEvent.idl
index cecf01e276a8a60d9b847d03143abcc30fe3a0df..604942fac47f59c7bc80f03d789205abba179b85 100644
--- a/third_party/WebKit/Source/core/events/PointerEvent.idl
+++ b/third_party/WebKit/Source/core/events/PointerEvent.idl
@@ -8,12 +8,12 @@
     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;
 };