Add UMA metric to track usage of sending a mousedown to select elements.

third_party/WebKit/LayoutTests/imported/wpt/pointerevents/pointerevent_support.js

Index: third_party/WebKit/LayoutTests/imported/wpt/pointerevents/pointerevent_support.js
diff --git a/third_party/WebKit/LayoutTests/imported/wpt/pointerevents/pointerevent_support.js b/third_party/WebKit/LayoutTests/imported/wpt/pointerevents/pointerevent_support.js
index 4067d53dd5da9a8d15341b686a299d98d62f4fd9..2f43118f783b3bd4e8412617236919a6e25f63dc 100644
--- a/third_party/WebKit/LayoutTests/imported/wpt/pointerevents/pointerevent_support.js
+++ b/third_party/WebKit/LayoutTests/imported/wpt/pointerevents/pointerevent_support.js
@@ -12,10 +12,12 @@ var All_Pointer_Events = [
 
 // Check for conformance to PointerEvent interface
 // TA: 1.1, 1.2, 1.6, 1.7, 1.8, 1.9, 1.10, 1.11, 1.12, 1.13
-function check_PointerEvent(event) {
-    var pointerTestName = event.pointerType + ' ' + event.type;
+function check_PointerEvent(event, testNamePrefix) {
+    if (testNamePrefix === undefined)
+        testNamePrefix = "";
+    var pointerTestName = testNamePrefix + ' ' + event.pointerType + ' ' + event.type;
     test(function () {
-        assert_true(event instanceof PointerEvent, "event is a PointerEvent event");
+        assert_true(event instanceof event.target.ownerDocument.defaultView.PointerEvent, "event is a PointerEvent event");
     }, pointerTestName + " event is a PointerEvent event");