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..bf316b24e00c8a874c11b8307a1a1fd0553e9bf9 100644
--- a/third_party/WebKit/Source/core/events/PointerEvent.idl
+++ b/third_party/WebKit/Source/core/events/PointerEvent.idl
@@ -8,7 +8,7 @@
     Constructor(DOMString type, optional PointerEventInit eventInitDict),
     RuntimeEnabled=PointerEvent,
 ] interface PointerEvent : MouseEvent {
-    readonly    attribute long      pointerId;
+    [MeasureAs=PointerEventCount]   readonly    attribute long      pointerId;

[tdresser, 2016/04/19 12:43:20]

To measure any usage of an interface, it looks like the most common approach is
to measure every attribute on the interface with the same use counter, e.g.,
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/WebKit....

[sahel, 2016/04/19 17:57:06]

Done.

     readonly    attribute double    width;
     readonly    attribute double    height;
     readonly    attribute float     pressure;